In a triangle the midline joining the midpoints of two sides is parallel to the third side and half as long ⇒
AС = 2*MN
3x + 10 = 2(x+5)
3x + 10 = 2x + 10
3x - 2x = 10 - 10
x = 0
M is a midpoint of AB ⇒
AM = BM
3y - 8 = 2(y+1)
3y - 8 = 2y + 2
3y - 2y = 2 + 8
y = 10