#### Answer

6

#### Work Step by Step

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.
$2a\div2=12\div2$
$a=6$