Answer
$\theta = 25.4^{\circ}$
Work Step by Step
Let $v_b$ be the speed of the motorboat in still water.
Let $v_c$ be the speed of the current.
Let $v_m$ be the actual speed of the motorboat. Note that the vector $v_m = v_b+v_c$
These three vectors form a right triangle where $v_b$ is the hypotenuse.
We can find $\theta$:
$sin~\theta = \frac{v_c}{v_b}$
$sin~\theta = \frac{3.0}{7.0}$
$\theta = arcsin(\frac{3.0}{7.0})$
$\theta = 25.4^{\circ}$
