Question

Angle RST is a right angle. Angle RSU has a measure of 25°.

Answer 1

Since angle RST is a right angle, it has a measure of 90°. 
Angle RSU could either be inside or outside the right angle. 
If Angle RSU is inside, the measurement of Angle TSU is:
90°- 25° = 65°

If Angle RSU is outside, the measurement of Angle TSU is:
90° + 25° = 115°

Answer 2

$\angle RST=90^{\circ}\\\\ \angle RSU=25^{\circ}\\\\ \angle RST=\angle RSU+\angle TSU\\\\ \rightarrow 25^{\circ}+\angle TSU=90^{\circ}\\\\ \rightarrow \angle TSU=-25^{\circ}+90^{\circ}\\\\\angle TSU=65^{\circ}$