Since both are y, you know the values are equal to each other.
3/2x - 6 = -9/2x + 21
3/2x = -9/2x + 27
Get rid of fraction (multiply by 2)
3x = -9x + 54
12x = 54, x = 4.5
Plug 4.5 into any equation as x
y = 3/2(4.5) - 6
y = 6.75 - 6 = 0.75
Solution: y = 0.75 or 3/4