The angles at the base are congruent, because the triangle is isosceles. The remaining angle is supplementary with 3x, so it is [tex] 180-3x [/tex]
So, the sum of the three angles is
[tex] (x+15)+(x+15)+(180-3x) = 210-x [/tex]
And the sum of the angles of a triangle is always 180:
[tex] 210-x = 180 \iff x = 210-180 = 30 [/tex]
The 3 angles in the triangle add up to 180 degrees.
So x + 74 + 73 + 37 + x = 180
2x = 180 - 74 - 73 - 37 = -4
x = -2 answer
3x = 2(x + 15)
x = 30 answer
