7 less than three fifths of b is a
'7 less than' means that it will be something minus 7, and that something is 3/5 of b
of usually means you multiply, so 3/5 of b is 3/5b
so far it's '3/5b-7'
'is a' means that the end result is 'a' that means '='
soo,
3/5b-7 = a
that's the answer.