One of the two forces acting on a body which is at equilibrium is $5\hat{i} - 7\hat{j}$. What is the value of the other force ?

$-5\hat{i} + 7\hat{j}$

For a body in equilibrium acted upon by two forces, the forces must be equal and opposite.

Let $F$ be the given force such that : $F = 4\hat{i} - 7\hat{j}$

The force that will be equal and opposite is thus, given by : $F' = - 4\hat{i} + 7\hat{j}$