Your last line is correct.
However, your 2nd line should start with a 36, not just a 3; and your third line should say y^2 = 81/36 (you take the square root in the next line).
Your fourth line should say y = ±9/6 as 9 is the square root of 81 and 6 is the square root of 36
Now 9/6 = 3*3/(2*3)
So y = ±3/2 (or ±1.5)
.