15 = 2x^2 - 6x + 8
0 = 2x^2 - 6x - 7
It doesn't factor nicely, so let's use the quadratic formula.
Note: This may be unfair since you probably haven't been given this formula yet. It lets you solve any 2nd-order (something with an x^2) equation.
(6 +- sqrt(36 + 56))/4
(6 +- sqrt(92))/4
(6 +- sqrt(4*23))/4
(6 +- 2sqrt(23))/4
(3 +- sqrt(23))/2
Answer: x = (3+sqrt(23))/2 or x = (3-sqrt(23))/2