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