Answer :

Lilith
[tex]5r^2 = 80 \ \ /:5\\ \\r^2=\frac{80}{5}\\ \\r^2=16\\ \\r^2-16=0\\ \\ r^2 - 4^2 =0 \\ \\(r-4)(r+4)=0[/tex]

[tex]r-4=0 \ \ \vee \ \ r+4 =0 \\ \\r=4 \ \ \vee \ \ r = -4 \\ \\ \\ a^2-b^2 = (a- b)(a+b)[/tex]