If angle p is a right angle and angle p and q from a linear pair, then angle q is

< P is a right anglea right angle is 90 degrees.
< P and < Q form a linear paira linear pair, when added, = 180 degrees

so if < P is 90and when added to < Q = 180...
90 + Q = 180
Q = 180 - 90
Q = 90 <== so < Q is also a right angle...measures 90 degrees