Question

3r^2-9r=12

Answer 1

move the 12 over, factor a 3 and solve

Answer 2

@Luigi0210 's solution works just fine, but if you come to an equation that doesn't factor nicely, you can subtract 12 so that your equation is equal to zero, then use the quadratic formula \[x=\frac{ -b \pm \sqrt{b^{2} -4ac} }{ 2a }\] where a=3 b=-9 and c=-12