calculating the gradient of a function

Hi,

I am ploting the function K (below) for a range of values of T, from 0.001 to 100000. On the logscale, the T is from -3 to 5.

K = (7213*tanh((393*Tc*(1/T + 4087/(10000*Tc)))/250))/(10000*Tc*(1/T + 4087/(10000*Tc)))

Need to find the value of Tc such that the gradient of the graph (K) on log(T) base is, say 4.

Can anyone give me some ideas or directions about how to do this?

I have tried some basic methods but have not had any success.

Thanks very much.