Factor r out of r2-4r.
Factor r out of r2.
Factor r out of -4r.
Factor r out of r⋅r+r⋅-4.
If any individual factor on the left side of the equation is equal to 0, the entire expression will be equal to 0.
Set the first factor equal to 0.
Set the next factor equal to 0 and solve.
Set the next factor equal to 0.
Add 4 to both sides of the equation.
The final solution is all the values that make r(r-4)=0 true.