It is bad practice, although it is often done, to use the same letter to represent the "dummy" variable of integration and as a variable outside the integral (as in one of the limits of integration).
If you meant
[tex]\int_x^0\frac{2mdt}{ct^2}[/tex]
Then rewrite the t2 in the denominator as t-2:
[tex]\frac{2m}{c}\int_x^0t^{-2}dt[/tex]
You still have a problem- that's an improper integral which will have serious difficulties at 0! This integral does not exist.