You can put this solution on YOUR website! With an equilateral triangle, all sides are equal, so a = b = c, where a = one side's length, b = another side's length, and c = final side's length.
The perimeter of a triangle = a+b+c = 10.5
OR
3a = 10.5
a = 3.5
which means
b = 3.5
and
c = 3.5
.
done