[![enter image description here](https://i.stack.imgur.com/k3ud5.jpg)](https://i.stack.imgur.com/k3ud5.jpg)

If the following graph is given then, why is the displacement equal to the area of the shaded triangle above the axis minus the are of the shaded triangle below the $t$ axis.

Why can it not be area of the shaded triangle above the axis minus the area of the trapezium

I get that it’s because we take the area with respect to $t$ axis. But I don’t understand why.