How do you make 'a' the subject of the formula: ab - cd = ac

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