Explanation:
it is a square.
so, all sides are equally long.
that means e.g.
y - 3 = 2y - 11
-3 = y - 11
8 = y
and then
3x - 10 = 3y - 19 = 3×8 - 19 = 24 - 19 = 5
3x = 15
x = 15/3 = 5
so the length of all 4 sides is by using e.g.
y - 3 = 8 - 3 = 5
each side is 5 units long.