completing the square
2m^2-m-15=0
using the quadratic formula
2y^2-3y-6=0
2m^2 - m = 15
m^2- (1/2)m = 15/2
m^2 - (1/2)m + 1/16 = 15/2 + 1/16
(m - 1/4)^2 = 121/16
m - 1/4= ± 11/4
m = 1/4 ± 11/4
m = 3 or m = -5/2
the second one is simply subbing into the formula and number-crunching.
let me know what you got.