What is the system's mean time to failure?
For part 1, its simple. You need to find the $x$ (as defined below) for which MTTF(system) = 1.20M.

So $1.20 * 10^6 = \frac{1}{[(\frac{1}{6*10^6})*x]} \implies x = 5$.

We can have 5 disks (50 TB) if we want an MTTF(System) = 1.2M.

Part 2 is homework for you.