Question

Mx = Nx - Pt isolate x

Answer 1

Mx = Nx - Pt

To isolate 'x', we have to group all terms that have 'x' together 
We have Mx on the left-hand side of the equation and Nx on the right.
Moving Nx to the left-hand side of the equation, we have:

Mx - Nx = Pt

From here we factorize 'x'

x (M - N) = Pt

Then we divide both sides by (M-N)

$x= \frac{M-N}{Pt}$