$P=\sqrt{ \frac{ mx-{t}^{3}x }{ t } }$
Swap the sides of the equation$\sqrt{ \frac{ mx-{t}^{3}x }{ t } }=P$
Square both sides of the equation$\frac{ mx-{t}^{3}x }{ t }={P}^{2}$
Multiply both sides of the equation by $t$$mx-{t}^{3}x=t{P}^{2}$
Factor out $x$ from the expression$\left( m-{t}^{3} \right)x=t{P}^{2}$
Divide both sides of the equation by $m-{t}^{3}$$x=\frac{ t{P}^{2} }{ m-{t}^{3} }$
Use the commutative property to reorder the terms$x=\frac{ {P}^{2}t }{ m-{t}^{3} }$