For (c), you are looking for the probability that , while eliminating the area of the graph up to .

which is

Convert to the Z-values and compute the probability.

Bear in mind that the probability of the bulb failing within the first 850 hours is now zero.

Hence your fraction has in the denominator.

For (d), use the Binomial conversions

where p=0.07, q=0.93, n=1200