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)

If you make it a little more clear i can answer it