Answer
$Q _{net} = 50 J$
Work Step by Step
For the full cycle, the $Q _{net} $ should be the whole area that the path covered which is the 'area of triangle'.
Area of triangle = $\frac{1}{2} \times height \times base$, so
$Q _{net} = \frac{1}{2} (350 K - 300K)(8.0J/K - 6.0 J/K) $
$Q _{net} = \frac{1}{2} (50K)(2.0 J/K) $
$Q _{net} = 50 J$
