Question 50127
ar+b=p [Original equation]
ar+b-b=p-b [Solve for r]
ar=p-b
ar/a=p-b/a
r=p-b/a

.
check by plugging (r=p-b/a) back into the original equatin

ar+b=p [Original equation]
a(p-b/a)+b=p
p-b+b=p
p=p [Checks out]