The continuous random variable Y is normally distributed with mean 15 and standard deviation 6
(a) Find P(X21)
(b) Find the value of d such that P(15<X<d)=0.3152
How do you answer part (b)?
How can an exam possbly test you on material that has not been taught? Unfortunately, there are many things the question you posted expects you to already understand. MHF cannot act as a surrogate teacher. If you have resolved to self-learn the normal distribution I suggest you obtain a decent textbook that covers the material and start from the very beginning.
I also suggest you discuss the issues raised in this thread with your teacher.