So the equation is the problem. But I don't know where the mistake is or is the approach wrong. I'm sure I solved it properly. It was t= 2*sin(10°)/9.81
I created a Vector which describes the direction of the medium. with r=(x(t),y(t))=((x0+cos(b)*t+0*t^2/2),(y0+sin(b)*t-9.81*t^2/2))
I created a vector for the acceleration a=(0,-9.81)
Now only the y(t) is relevant since x(t)
acceleration is zero. sin,cos describe the direction of the velocity...