Substituting a Value

Evaluating an expression means finding its value for a given number. Take 2n + 5 when n is 7. Write 7 in place of n. This is called substituting. Since 2n means 2 times n, the expression becomes 2 × 7 + 5.

Now the order of operations from Section 1 takes over: multiply first, so 2 times 7 is 14, then add 5 to get 19. On the board, name the value with a letter, v: write v = 2 × 7 + 5, then v = 19 on the next line.

When you substitute, put the number in parentheses in your head or on paper: 2n becomes 2(7). The parentheses remind you that the number is multiplied, never glued on. 2n with n = 7 is 14, not 27. They matter even more in the next lesson, when the number is negative.