Question

What is the length of MN

Answer 1

It is an isosceles triangle and therefore

3x = x + 10
2x = 10
x = 5
Since x = 5 and MN = x + 10 then,
MN = 15.

Answer 2

Because this is an isosceles triangle, 3x = x + 10.  Thus, 2x + 10 and x = 5.

With x=5, MN has the length 5+10, or 15.