ax + b = a(x + c)
distribute the a to both x and c
a(x + c) = ax + ac
ax + b = ax + ac
subtract ax from both sides
ax (-ax) + b = ax (-ax) + ac
b = ac
however, you are finding the value of c
b = ac
isolate the c by dividing a from both sides (because of equal sign, what you do to one side you do to the other)
b/a = ac/a
c = b/a
c = b/a or D) should be your answer
hope this helps