20-x2=5x, x2+5x=20,
x2+2(5x/2)+(5/2)2=20+(5/2)2,
(x+5/2)2=20+25/4=105/4,
x+5/2=±√105/2, x=-5/2±√105/2, so x=-5/2+√105/2 or -5/2-√105/2. (√105=5.1235 approx.)
This result is symbolised by the x-intercepts on a graph of y=x2+5x-20 (parabola). The intercepts lie at equal distances from the axis of symmetry, x=-5/2.
Another way to symbolise this is to draw a graph of y=x2+5x and a horizontal line y=20. Where the line cuts the curve in two places are the two solutions for x.