The magnetic field B crossing normally a square metallic plate of...

From the graph, it can be seen that the value of the magnetic field at $t = 2s$ is $4mT$, whereas the magnetic field at $t = 4s$ is $8mT$.

The formula to calculate the magnitude of the induced emf is given by

$\begin{cases}{ε}_{\text{ind}} & = & \dfrac{dϕ}{dt} \\ & = & \dfrac{d}{dt}\left(BA\right) \\ & = & A\dfrac{dB}{dt} . . . \left(1\right)\end{cases}$

$\dfrac{dB}{dt}$ is the slope of the given graph.

Hence, the value of the induced emf can be calculated as follows:

$\begin{cases}{ε}_{\text{ind}} & = & 4{m}^{2} \times \left(\dfrac{8mT - 4mT}{4s - 2s}\right) \\ = & 8mV\end{cases}$