Solve variation equations

Question

The quantity r varies directly with s and inversely with t. When s= -5 and t= -1, r= 10 Find r when s=3 and t=-1

Answer 1

rt = ks

(10)(-1) = -5k

(-2)(-1) = k

2 = k

(r)(-1) = (3)(2)

Can you solve for r and answer?