In a normal Distribution 31% of the item are under 45 and 8 % are above 64.Find its mean and SD?

In a normal Distribution 31% of the item are under 45 and 8 % are above 64.Find its mean and SD

My doubt is how to know if z < 0 or >0 .

I really need to understand the concept behind taking z1. What does 31 % indicate? negative as it is below 50 % ? Somebody please help me.I'm badly in need of a way to solve this problem