Newton's second law is:
F=ma,
where a=dv/dt, so
F=m(dv/dt)
Rearranging gives:
Fdt=mdv.
Basic integration gives:
Ft=m(vf-v0),
where vf and v0 are the final and initial velocities of the object respectively.
In your case vf=0, because the ball stops completely, and v0=10m/s.
Rearranging the last expression gives F=(m(vf-vo))/t.
Plug in numbers to find F=(210)/0.03=666.6 N
