@acer I was trying to calculate the starting velocity (v0) of a pingpong ball, when it's fired from a machine. For that i needed to calculate the t (time in s) of the flight curve (until the pingpong ball hits the ground), with displacementx (verplaatsingx) of l (2 meter). 

@tomleslie has solved the issue I had by replacing the Pi. No idea why Pi won't solve by itself though.

I added myPi:=evalf(Pi) and replaced Pi with myPi. 

It now works, Thank you!

Thank you for your fast reply :)

I removed the index, but I now get the error that fsolve can't determine if the expression is true or false.

Error, (in fsolve/checkrange) cannot determine if this expression is true or false: abs(9.486949280+146.0152844220*Re(cos(1.156627765580*_Z)/sin(1.156627765580*_Z)))+146.0152844*abs(Im(cos(1.156627765580*_Z)/sin(1.156627765580*_Z))) <= 0.7589559424e-8

I'm quite new to Maple, so I don't really understand how to solve this. How do I get a value for v0?