Answer
The third angle is $45^{\circ}$.
Work Step by Step
The measures of the three angles of a triangle equal $180^{\circ}$; therefore, if we have the measure of two angles, we can figure out the measure of the third angle.
$45 + 90 + x = 180$
Add the constants on the left side together:
$135 + x = 180$
Subtract $122$ from each side to solve for $x$:
$x = 45$
The third angle is $45^{\circ}$.
