Doubling average speed over two one-mile laps

A driver covers the first lap of a one-mile circuit at a steady $30$ mph. At what constant speed $x$ must the second lap be driven for the average speed over both laps to be $60$ mph? Enter $-1$ if no such speed exists.

$-1$. Averaging $60$ mph over $2$ miles needs a total time of $2$ minutes, but the first mile at $30$ mph already uses $2$ minutes. The second lap would have to take zero time, so no finite speed works.