How to calculate the emf with flux density given instead of flux
A cross sectional coil area of 10 square cm is 0.001 square metres.
A flux reversal from +0.4 T to -0.4 T is a change of 0.8 T
This means that \$\frac{d\Phi}{dT}\$ = 0.001 x 0.8/0.02 = 0.04 webers per second
The induced voltage is 2000 x 0.04 volts = 80 volts.


Given what you have put in your question there is no way of saying whether the voltage is negative or positive.