This problem says nothing about the height of the tent, so I must assume that the "equilateral triangle" is the BASE of the tent.
Since the 3 sides of this base are equal, the perimeter is 3(x-10x - 10) units, and this equals the given 63 units.
Thus, 63 = 3x - 30x - 30, or 93 = -27x. Thus, x = -93/27, or -31/9.
There may be a problem here with the expression you typed in:
x - 10x -10
because x - 10x = -9x, so there is no reason to type in x-10x.
Double check to ensure that you have correctly typed the original problem statement.