Show that if y=mx+c is a tangent to the circle x^2+y^2=r^2,then c^2=r^2(1+m^2)

Cindi bought a sheet pizza for a party. some of the pizza has pepperoni, some has onions, and the rest of it has both. two-fifths of the pizza has just pepperoni and one-third of the pizza has just onions. what fraction of the pizza has both pepperoni and onions? answer asap