To complete the square, add the square of half the x-coefficient.
.. x^2 +3x +2.25 = 12.25
.. (x +1.5)^2 = 3.5^2
.. x +1.5 = ±3.5
.. x = -1.5 ±3.5
.. x = -5 or 2
You can also solve this by factoring.
.. x^2 +3x -10 = 0
.. (x -2)*(x +5) = 0
.. x = 2 or -5