## Geometry: Common Core (15th Edition)

Let a be the measure of the angle. By the definition of complement, the measure of the complement is $90-a$. Restate the sentence as an expression. $a=(90-a)-78$ Simplify. $a=12-a$ Add a to each side. $a+a=12$ $2a=12$ Divide each side by 2. $2a\div2=12\div2$ $a=6$