When there's more than one letter, and you just have to solve for one variable, you can just solve it like you would any other equation. Treat the other variables like numbers and add, subtract, multiply, and divide them to both sides of the equation in order to isolate the variable you want to solve for.
[tex]t= \frac{pd}{2x} \\ \\ t*2x= \frac{pd}{2x}*2x \\ \\ 2tx=pd \\ \\ \frac{2tx}{2t} = \frac{pd}{2t} \\ \\ x=\frac{pd}{2t} [/tex]
