Calculus Problem?

Snow began to fall on morning of FEB 2nd. and continued into the afternoon. At noon a snowplow began to remove the snow from the road at a constant rate. The plow traveled 6km from noon to 1 pm but only 3 km from 1–2 pm. When did the snow begin to fall.

t= time measured after noon in hours.
x(t)= distence traveled by plow at time t

speed= dx/dt

b= number of hours before noon it began to snow

find an expression for the height of the snow at time t.

Then find the R (rate of removal) in m^3/h- which is constant. Or prove that it is constant.

How in the world do I solve this? Any help would be appreciated

Thanks!