Random variable X ~ N(20,12)

Find the values of a and b:

P(X<a) = 0.40

P(X>b) = 0.6915

I can't get the answer in the text book and no worked examples are available. Any ideas would be greatly appreciated ...

My workings are:

P(X<a) = P(Z<((a-20)/(sqrt12)) using mu and sigma

P(Z<((a-20)/(sqrt12)) = 0.40

but dont have 0.4 in my Z tables.

tre same for the other equation.