check the picture below.
we know TQ is 18, and notice those red tickmarks, meaning 2x + 10 is also 18, namely 2x + 10 = 18, 2x = 8, x = 4.
now, the angles at point T are both 90°, the sides RT = TQ, the sides TS = TS, namely, the triangle on the left is congruent with the triangle on the right, due to Side Angle Side, SAS. If both triangles are congruent, then RS = QS, as you see in the picture, and 9x - 11 is just 9(4) - 11 = 25.
