-x^2 -3x = x + 3
make one side of the equation equal to zero. I suggest to always try to keep any squared numbers positive to make it easier on yourself
Add -x^2 and -3x to both sides
-x^2-3x = x+3
+x^2 +3x +x^2 +3x
0 = x^2 + 4x + 3
factor
(x +3)(x+1)
x = -3 x=-1