An equilateral triangle has all sides equal so that if one side is x, the other 2 sides are equally x
so perimeter is addition of all sides of the triangle
and that will be x +x+x = 3x
3x= perimeter------(1)
from the question the perimeter = 51 inches-----(2)
so that equating 1 and 2 yields
51 inches = 3x
51/3 = 3x/3
x =17 inches