1/T - 1/M = 1/R
Add 1/M to both sides, subtract 1/R from both sides:
1/T - 1/R = 1/M
Take reciprocal of both sides:
1/(1/T - 1/R) = M
Re-writing with M on LHS:
------
Just in case the student is wondering, the other tutor's answer is just a different form for the one I provided:
=
=
Also note the implied restrictions:
,
,