Simple system of equation

Bob has $84 consisting of $1 and $5 bills. He has 6 more $1 bills than he does $5 bills. How many $1 bills does he have?

I'm having trouble working this problem out...can you please show the steps?

Thanks

Let call x be the $5 bill
then the $1 bill is x + 6

5x + (x + 6) = 84
6x = 78
x = 13

Bob has 13 bills of $5 and 19 bills of $1