Hey there!!
Closely observe the triangle, the triangle given is a isosceles triangle , hence, the angles on the base would be equal and hence, angle R = 3x + 30
The sum of all angles in a triangle = 180
Hence , angle R + angle S + angle T = 180
... 3x + 30 + 12x - 15 + 3x + 30 = 180
... 18x + 45 = 180
... 18x = 180 - 45
... 18x = 135
... x = 135 / 18
... x = 7.5
Now substitute the value of x in the angle formula of R and we get the angle R.
... 3x + 30
... 3 ( 7.5 ) + 30
... 22.5 + 30
... 52.5° is the answer
Hope my answer helps!