How to solve this system?

[Helge Gabert]

Where is the problem?

If m1, m2, r not equal zero, just take the inverse and multiply out.

p1=(-m2+r)/(m1*r+m1-m2*r-m2)
p2=(m1-r)/(denom. as above)

and observe r>m2 and m1>r (which is NOT what you wanted, but you can check various combinations of negative numerator divided by negative denominator and see if find combinations which satisfy m1<r<m2).