I thought the previous explanation posted was quite confusing so I tried the question out myself:
LET:
L= Angels RSQ & RQS (equal sides of a triangle)
M= Angels TSU & SUT (equal sides of a triangle)
R= Angel PRT
T= Angel RTP
X= Angel QSU
So looking at line RT we have:
L + X + M = 180
Looking at triangle RPT we have:
90 + T + R = 180
Looking at triangle SUT we have:
2M + T = 180
Looking at triangle RQS we have:
2L + R = 180
All together:
1. L + X + M = 180
2. 90 + T + R = 180
3. 2M + T = 180
4. 2L + R = 180
Simplify 2:
T + R = 90
Combine 3 & 4 to get:
2M + 2L + T + R = 360
2M + 2L = 360 - 90 (substitute 2)
2M + 2L = 270
M + L = 135
now taking this equation and substituting it in 1gives:
X + 135 = 180
X = 45