Answer: 8

Divide the second equation by $2$: $y = \frac{k}{2}x + 5$. A system of two linear equations has no solution when the lines are parallel and distinct, with equal slopes and different $y$-intercepts. The $y$-intercepts, $-3$ and $5$, are already different, so set the slopes equal: $\frac{k}{2} = 4$, which gives $k = 8$.

Answering $4$ matches $k$ to the first slope without first dividing the second equation by $2$.