m=u/k-r/k for k

[math_celebrity]

m=u/k-r/k for k

Multiply both sides by k to eliminate the k denominator:
km = uk/k - rk/k

Cancel the k's on the right side and we get
km = u - r

Divide each side by m:
km/m = (u - r)/m

Cancel the m on the left side:
k = (u - r)/m