Question

An equilateral triangular tent has sides that measure x−10 units. Its perimeter is 63 units.

Answer 1

An equilateral triangle's sides are all equal. So you can set up the following equation and solve.

3(x-10)=63
x-10=21
x=31

Answer 2

An equilateral triangle has 3 equal sides. The perimeter would equal all of those sides added together. So divide 63 by 3
63/3=21
21 is equal to one side. One side is equal to x-10. So set them equal to each other 
21=x-10
solve
x=31