so you have to try and elimate everything by getting a by itself. so u move all the variables with an 'a' in it. so thats ax-aw=-z-y ( dont forget to change the signs since u r moving it to the opposite side of the equation) then get 'a' by itself so its a(x-y)= -z-y then move (x-y) to the other side but now its the denominator.
so ur answer is a= -z over -x (since the y's cancel)