Try to get any variable that involves with "t" to one side.
bh+rt=mn-vt
Add "vt" to both side
bh+rt+vt=mn
Subtract "bh" from both sides
rt+vt=mn-bh
We want to get "t" so we factor it the variables with "t"
t(r+v)=mn-bh
Divide both sides by "r+v"
t=(mn-bh)/(r+v)