angle QRS is equal to 135 degrees, as both angles are alternate, or you could do 180 degrees - 45 degrees. for angle RST you would get 45 degrees, as again it is alternate or because you subtract 135 from 180.
from here you add the angles:
105+135+45= 285
so, subtract 360-285=75. since this question is asking the value of x, divide 75 by 3, which gives you the answer 25.
x=25