Make a the subject of the following:

a-3r=r (a+1)

A-3r=r (a+1)

add 3r to both sides

a=r3+r (a+1) to both sides

undistribute since ab+ac=a (b+c)

a=r (3+a+1)

add like terms

a=r (4+a)

distribute

a=4r+ra

subtract ra from both sides

a-ra=4r

undistribute a

a (1-r) = 4r

divide both sides by 1-r

a=4 / (1-r)