Make X be the first variable (the life expectancy that is normally distributed).
The thing is to make X be Z, where Z is a standard normally distributed variable. And we know that Z=(X-48)/8.
Then use the z-table like you did in the first question (don't laugh, I didn't understand what you wrote in a first time lol)
In the second question, you're asked to find z such that P(Z>z)=90%, then use the definition of the standard deviation.