The base angles are opposite the congruent sides (marked by the single tickmarks). The base angles are
Angle R = 2x
Angle Q = 2x
Add up the three angles, and set the sum equal to 180. Then solve for x
P+Q+R = 180
40+2x+2x = 180
40+4x = 180
4x+40 = 180
4x+40-40 = 180-40
4x = 140
4x/4 = 140/4
x = 35
So the value of x is 35 which means
Angle R = 2*x = 2*35 = 70
Angle Q = 2*x = 2*35 = 70
Notice how
P+Q+R = 40+70+70 = 110+70 = 180
which helps us confirm the answer
