That's not a perfect square trinomial. You can't complete the square because of the 2 coefficient in front of the x^2, so you do have to use the quadratic formula, which from standard form ax^2+bx+c=0,
-b+or-[sqrtb^2-4ac]/2a So let's plug in the information:
-(-16)+-[sqrt-16^2-4(2)15]/2(2)
16+-[sqrt256-120]/4
16+-(sqrt136)/4
16+-(11.66)/4
16+-2.915
x= 18.915 or
x=14.915 You just have to plug these values back into your original equation to see if one or both are true.
http://www.purplemath.com/modules/solvquad4.htm
wpf