percentages forumla - method to solve?

I've forgotten how to do math! It's been so long... this is embarassing. I have an amount of money, $7000 for example, that is the total amount spent for an advertising campaign. The campaign total cost includes the cost of the ads, and then the agency fee which is 20% of the cost of the ads.

X = total cost of campaign

B = cost of ads

C = 20% of B

B + ( .2 x B) = 7000

how would I use a forumla to calculate this? Would anyone be able to send me some links or point me in the right direction?