Renting the Right Number of Machines

A team runs one training job in the cloud on $m$ rented machines. Renting costs $10 per machine, while the bill for running time falls as more machines share the work. Their total cost in dollars is

$C(m) = \frac{9000}{m} + 10m, \qquad m > 0$

Which value of $m$ solves $\min_{m} C(m)$?