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All courses Math Foundations · MF.EQ.2 29 of 60
Evaluate algebraic expressions correctly using substitution and order of operations.

Substitute into a one-variable expression

Problem
Evaluate 3x + 2 when x = 4.
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Hint

Start by replacing x with 4 in the expression: rewrite 3x + 2 as 3(4) + 2.

Substitution means use the given value everywhere the variable appears, and 3x means 3 times x, not 3 plus x.

Solution walkthrough

01

Substitute the given value for x

\[3x~+~2~=~3(4)~+~2\]

To evaluate the expression, replace x with 4 everywhere x appears. Here, the only x is in 3x.

02

Multiply first

\[3(4)~+~2~=~12~+~2\]

The expression 3(4) means 3 times 4, so it becomes 12.

03

Add the remaining terms

\[12~+~2~=~14\]

After the multiplication, just add 2 to 12.

04

State the value of the expression

\[3x~+~2~=~14~\text{when}~x~=~4\]

So the value of the expression is 14.

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Another way

  1. Think of 3x as 3 groups of x. With x = 4, that is 3 groups of 4, or 12, and then 12 + 2 = 14.

!

Common mistake

A common mistake is treating 3x as 3 + x and doing 3 + 4 + 2 = 9. In this problem, 3x means 3 multiplied by x, so you must do 3(4), not 3 + 4.