you first have to expand it,
f(x)=-2(x²+3x-5x-15)=-2x²+4x+30
the line is symmetry is x=-b/2a
in this case, b=4, a=-2,
x=-4/-4=1
a quicker way to do it: the two zeros for -2(x+3)(x-5) are x=-3 and x=5
the point equal distance to -3 and 5 is at x=1
therefore, the symmetry line is x=1
