Hey nyago.

The thing you will have to do is use the information about the distribution to obtain the probability.

To look at failures before 9000 hours, we need to look at the probability where P(X < 9000) where X is the distribution for failures.

It says the distribution is symmetric and mound shaped which suggests a hint to use the normal distribution. It also gives you the mean and standard deviation.

So given X ~ Normal(7800,1200^2) [mean,variance] can you find P(X < 9000)?