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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

\[3x~+~2~\text{with}~x~=~4~\text{becomes}~3(4)~+~2\]

Replace the variable x with 4 while preserving the implied multiplication.

02

Multiply before adding

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

Order of operations evaluates the product first.

03

Finish the evaluation

\[14\]

Adding 2 to 12 gives the value 14.

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Another way

  1. Three groups of four total 12; two more gives 14.

!

Common mistake

Do not treat 3x as 3 + x or add 2 before completing the multiplication.

Solution walkthrough video