Chapter 2 - Solving Equations - 2-1 Solving One-Step Equations - Practice and Problem-Solving Exercises - Page 85: 15

a=26

In order to solve this algebraic expression, we must isolate the variable. A variable is the letter in the problem (for instance, in the equation x+2=10, x is the variable). Because addition and subtraction are inverse operations (they cancel each other out), we know we must add when a number is being subtracted from the variable and subtract when a number is being added to the variable. In the equation a-5=21, we must get rid of the 5, which is on the same side as a, in order to solve. Thus, we add 5 on both sides of the equation to find that a=26. We plug 26 in for a in our original equation and confirm that our answer is, indeed, correct.