Calculation given VFE KFE x L Length of line x ddt x VSttd

Calculation given:

VFE = KFE x L (Length of line) x d/dt x (VS(t-td)

I need to calculate VFE but do not know what figures to use in the place of d/dt.

I have:

*Length of line = 50mm

*Voltage step = 2V

*VS = 0.99V

*t = 100 ps

*td = 284 ps

KFE = -212 x 10^-12

ZO = 70.7 Holmes

VP = 176 x 10^6

Mutual Inductance = 80 x 10^-9

Mutual Capacitance = 10 x 10^-12

Line Inductance = 400 x 10^-9

Line Capacitance = 80 x 10^-12

Differentiation of any constant is =0

d/dt =where t =100ps

so d/dt is 0