Let's do it step by step:
"three-fifths of a" would be 3/5 multiplied by a, as that is how you find fractions of numbers, so we can write that as (3/5)a
"7 less" means we have to subtract 7 from (3/5)a, so we get (3/5)a - 7
Now all we need to do is make it equal to B, so your final answer is
(3/5)a - 7 = b
I hope this helps! I put the 3/5 in brackets just so you didn't mistake it as 3/(5a) :)
