Answer
$72^{\circ}$
Work Step by Step
We can find the required angle as follows:
$x^{\prime}=\sqrt{1-v^2/c^2}$
We plug in the known values to obtain:
$x^{\prime}=3.0\sqrt{1-(0.90)^2}$
$x^{\prime}=1.3m$
Now, $tan\theta=\frac{y}{x^{\prime}}$
$\implies \theta=tan^{-1}\frac{y}{x^{\prime}}$
$\theta=tan^{-1}\frac{4.0}{1.3}=72^{\circ}$
