Assuming that on average, mathematics textbooks and physics textbooks have the same number of pages, what is the probability of picking a sample of 20 physics textbooks and of 25 mathematics textbooks and getting a sample mean so much higher for the physics textbooks (p-value, to four places)?

I'm not sure if what I'm doing is correct. Can you let me know if I'm doing anything anything wrong?

standard deviation of physics/ mathematics textbooks: 50

x1 = 435 x2= 404 n1= 20 n2 =25

standard deviation=

square root (sd ^2)/ (n1) + (sd^2)/(n2)

square root ( 50^2)/ (20) + (50^2)/ (25) = 15

z = standard deviation/ x1 - x2

z= 15/ 435-404

= .9803

I'm not sure what to do after this, or even if I approached this problem correctly.

Please Help?