I guess this should be c^2-5c-24=0=(c-8)(c+3) so if it were to be solved c=8 or -3. On a graph we have a parabola that cuts the horizontal axis (c) at -3 and 8. The axis of symmetry lies midway between at the average of these two values: c=(8-3)/2=2.5 so the axis of symmetry is a vertical line c=2.5, which acts like a mirror reflecting both symmetrical halves of the parabola.