I cannot tell whether the comma in "9.81" is a punctuation comma -- which would be a syntax error in your expression -- or whether, instead, it's supposed to be a decimal point, which by default you'd need to enter as 9.81 in Mathematica. I suspect the latter, since I suspect you're referring to average gravitational acceleration on surface of Earth,

Offhand, the differential equations as given look quite nasty, and I would not expect an exact symbolic solution -- even after you replaced the approximate number 9.81 by the exact rational 981/100.

So you should try to do a numerical solution, and for that you'll need some initial conditions.