7m+3b=4700
If you multiply 4700 with 5 (take the answer as y), you will get the answer for 35m+15b.
3m+5b=4800
If you multiply 4800 with 3 (take answer as x), you will get the answer for 9m+15b.
Then, you subtract x by y (take answer as a).
Then, divide a by 32 to find m.