First bring all terms in 'a' to the left side of the formula by subtracting ac from both sides
ab - ac - cd = ac - ac ab - ac - cd = 0 now add cd to both sides ab - ac -cd + cd = cd ab - ac = cd now factor the left side by taking out the 'a' a(b-c) = cd now divide both sides by (b-c) a = cd / (b-c) done
