Answer:
3m - 30
Hope you could get an idea from here.
Doubt clarification - use comment section.
So the expression till now
[tex]\\ \sf{:}\Rrightarrow m(3)[/tex]
[tex]\\ \sf{:}\Rrightarrow 3m[/tex]
But the required weight is 30pounds less than it
[tex]\\ \sf{:}\Rrightarrow 3m-30[/tex]