Multiple choice
1) Solve 2x - 4y = 7 for x
To get x by itself, what is the first step?
1.
2.
3.
4.
Algebra 1 A
Chapter 2
Section 2.8
Solving equations with multiple
variables
Add 2x
Subtract 2x
Add 4y
Subtract 4y
Answer Now
D U
1) Solve 2x - 4y = 7 for x
Use a DO-UNDO chart to help determine the steps
Ask yourself,
• What is the first thing
we are doing to x?
• What is the second
thing?
Complete the undo column
by writing the opposite
operations in opposite
order.
DO UNDO
·2 ÷2
-4y + 4y
Follow the steps in
the ‘undo’
column to isolate
the variable.
1.
2.
3.
1) Solve 2x - 4y = 7 for x
+ 4y + 4y
2x = 7 + 4y
Add 4y to both
sides
2
2
Simplify
7  4y
Divide both sides
x
by 2
2
4. Does it simplify?
D U
2) Solve 2x - 4y = 7 for y
To get y by itself, what is the first step?
Add 2x
Subtract 2x
Add 4y
Subtract 4y
Answer Now
÷2
+ 4y
This fraction cannot be simplified
because both terms in the numerator
are not divisible by 2.
Multiple choice
1.
2.
3.
4.
·2
-4y
1.
2.
3.
4.
2) Solve 2x - 4y = 7 for y
- 2x
- 2x
Subtract 2x from
-4y = 7 - 2x
both sides
Simplify
-4
-4
Divide both sides
7  2x
by -4
y
Does it simplify?
4
Nope!
· -4 ÷ -4
+2x - 2x
3) Solve
ya
 c for y.
3
D U
3) Solve for y:
What is the first step?
1.
2.
3.
4.
Multiply by 3
Divide by 3
Add a
Subtract a
Answer Now
1. Clear the fraction
– multiply both
sides by 3
2. Simplify
3. Subtract a from
both sides
4. Simplify
3
*
+a
ya
 c ÷3
3
ya
c *3
3
y + a = 3c
-a -a
y = 3c - a
-A
·3
D U
· -2 ÷ 2
+ 4x -4x
4) Solve for y: 4x – 2y = 12
Multiple choice
1.
2.
3.
4.
Y= -4x + 12
Y= 4x - 12
Y = -2x + 6
Y = 2x - 6
1.
2.
3.
4.
Answer Now
4) Solve for y: 4x – 2y = 12
- 4x
- 4x
Subtract 4x from
both sides
-2y = 12 – 4x
Simplify
-2
-2
Divide both sides
by -2
y = -6 + 2x
Does it simplify?
or
y = 2x - 6
Yes!
Multiple choice
Multiple choice
5) The formula for the volume of a rectangular
prism is V = LWH. Which equation solves the
formula for L?
1. L = V - WH
VH
W
V
W
3. L 
H
4. L  V
HW
2.
L
6) The formula for the volume of a pyramid
1
is V = bh . Which equation solves the
3
formula for h?
1. h = 3Vb
3b
V
3V
3. h 
b
4. h  V
3b
2. h 
Answer Now
Answer Now
Example 7
The formula for the perimeter, P, of a rectangle is
P = 2L + 2W, where L is the length and W is the width.
Solve the formula for the perimeter of a rectangle for L.
P = 2L + 2W
P – 2W= 2L + 2W  2W
P – 2W= 2L
P – 2W= 2L
P  2W 2 L

2
2
P  2W
L
2
do
undo
*2
+2W
÷2
-2w
L
W