The first thing to know is that a square has equal sides, and that the area of a square is length x length.
Therefore in this case
Length x length = 100 or Length2 = 100
ans so Length = square root of 100 = 10
Now we can say that 4(3x - 2) = 10
Multiply out the brackets 12x - 8 = 10
Add 8 to both sides 12x = 18
Divide both sides by 12 x = 18/12 = (18/6)/(12/6) = 3/2 = 1.5
Now we can substitute in this value for x into our other expession for the length.
8x - y = 10
8(1.5) - y = 10
12 - y = 10
Add y to both sides 12 = 10 + y
Subtract 10 from each side 2 = y
So we have x = 1.5 and y = 2