Can anyone give me any hints on the following question:

Temperature of a thin slab exposed to convective heating at both sides varies with time acording to the following expression:

T(infinity) - T

T (infinity) - To = exp(-ht/rpc)

Where To and T (infinity) refer to intital and final (gas stream) temperatures respectively and 'r' is the thickness of the slab. Obtain the ignition 't' in terms of other variables using logarithmic laws. Discuss what factors t is directly and indirectly proportional to.