Answer
To travel straight across the river, the boat must be pointed upstream at an angle of $26.7^{\circ}$
Work Step by Step
In order to travel directly across the river, the upstream component of the boat's velocity relative to the water must be equal in magnitude to the water current of $1.8~km/h$.
Let $\theta$ be the angle at which the boat is pointed upstream. We can find the angle $\theta$:
$(4.0~km/h)~sin~\theta = 1.8~km/h$
$sin~\theta = \frac{1.8~km/h}{4.0~km/h}$
$\theta = sin^{-1}(\frac{1.8~km/h}{4.0~km/h})$
$\theta = 26.7^{\circ}$
To travel straight across the river, the boat must be pointed upstream at an angle of $26.7^{\circ}$
