Start with an equation summing all the angles in this triangle:
180 = <M + <N + <P
we are given <M and <N but not <P. But, since MN=NP, the angle <P is the same as the angle <M (isosceles, make a drawing to see). So
180 = 2<M + <N
180 = 2(3x+1) + x-11
180 = 7x - 9
x = 27
<P = 3*27+1 = 82 degrees
