You have the following equation:
M = 2rt³ - 3rx
In order to solve the previous equation for x, proceed as follow:
M = 2rt³ - 3rx subtract 2rt³ both sides
M - 2rt³ = -3rx divide by -3r both sides
(M - 2rt³)/(-3r) = x simplify left side
-M/3r + 2/3 t³ = x
