Answer
$$10,\ or, \ 3
.$$
Work Step by Step
For the equation $3x^2 -39x+90=x^2 -13x+30$, we use the quadratic formula as follows
$$ x=\frac{-b \pm \sqrt{b^{2}-4 a c}}{2 a}, \\
a=1, \quad b=-13, \quad c=30.$$
That is,
$$ x=\frac{13\pm \sqrt{13^2-4(1)(30)}}{2(1) }=10,\ or, \ 3
.$$
