Question

You have to get the R by itself

Answer 1

A = r/2L

A(2L) = r/2L(2L)       (multiply 2L to both sides to isolate r)

A(2L) = r   is your answer

hope this helps

Answer 2

If A = r / 2L, multiply both sides by 2L. 

2L cancels itself out and the new equation is (A)(2L) = r.