Once you can turn words into an expression, a letter stands in for a number you don't yet know. Substitution is the step where you finally do know it: you replace each letter with its given value and work the expression out like ordinary arithmetic.

Say x = 4. Then everywhere you see an x, write 4 instead:

Notice the last two steps follow the order of operations — the multiplication 3 \times 4 happens before the + 2. Substitution doesn't change those rules; it just gives you numbers to apply them to.

One letter at a time

Press play to watch a letter get replaced by a number, then the expression collapse to a single answer (a fresh value each time):

More than one letter

With several letters, substitute them all, then evaluate. If a = 5 and b = 2:

Be careful with the powers and any negative numbers — substitute first, then let the order of operations finish the job. This is exactly how you evaluate a formula: plug the measurements in for the letters and compute.

Khan Academy walks through evaluating an expression by substitution here: