The first triangle is an Isosceles triangle:
2 equal sides
2 equal angles ( opposite to the sides)
The sum of all interior angles of a triangle is 180°
So, for the left triangle:
51+x+x2 = 180
x+x2 = 180-51
x+x2 = 129
Since both angles are equal:
129/2 = 64.5°
x= 64.5°
x+y = 90°
64.5 + y = 90
y= 90-64.5
y= 25.5°
Right triangle:
25.5+25.5 +z = 180
z= 180-25.5-25.5
z= 129°