Hi, I would like to answer you question to help you out but unfortunately there is not enough information for me to do so. Maybe if you put a picture along with those words then I could solve the problem.
For example, when stating a problem such as 2 plus?= 4
Then I am able to solve by doing...
4-2=2
Check:
2 plus 2=4
But with your problem, F is perpendicular to line m whats blank
Then the only thing I am able to do is...
F=m
blank=?
Like I said I would love to help but I need a little more info than what is present now. Thanks.