Notebook sold Tk 36 → loss; sold Tk 72 → profit equal to twice the loss. CP?

Let the cost price be $x$. The loss is $x - 36$ and the profit is $72 - x$, and the profit equals twice the loss: $72 - x = 2(x - 36)$. Solving, $144 = 3x$, so $x = \text{Tk } 48$.