Answer
90$^{\circ}$
Work Step by Step
When the sum of the measures of the two positive angle is 180$^{\circ}$, the angles are called supplementary and the angle are supplement to each other.
a$^{\circ}$ and b$^{\circ}$ are supplementary angles therefore a$^{\circ}$ + b$^{\circ}$ = 180$^{\circ}$
If a is supplement to itself than b$^{\circ}$ = a$^{\circ}$
The supplement of angle a$^{\circ}$ is b$^{\circ}$which can be written as b$^{\circ}$ = 180$^{\circ}$ - a$^{\circ}$
Hence a$^{\circ}$ = 180$^{\circ}$ - a$^{\circ}$
a$^{\circ}$ + a$^{\circ}$ = 180$^{\circ}$
2a$^{\circ}$ = 180$^{\circ}$
a$^{\circ}$ = 90$^{\circ}$
