Solve by completing the square

x^2+3/2x=3

The "square" would be x^2 + 3/2 x + 9/16, so add 9/16 to both sides of the equation and then take the square root of both sides.

The first step will give you
(x + 3/4)^2 = 3 9/16 = 57/16

x + 3/4 = +/- sqrt(57/16)

x = -3/4 +/- (1/4)sqrt57