Hello to know the angle A and the value Y you need to know that the triangle in the image is a triangle isosceles, that triangle has 2 equal angles, so B is equal to C, then:
C = 25 °
B = 25 °
to know A it is necessary to add the two angles that we already know C + B
25 + 25 = 50 and we subtract it wit 180
180-50 = 130 Then A = 130 ° but A = 3y + 70 then clear to Y:
A = 3Y + 70
130 = 3Y + 70
130-70 = 3Y
60 = 3Y
60/3 = Y
20 = Y
I hope It help you
