Abird flies 2.8 due west and then 2.7 due north. another bird flies 2.7 due west and 2.8 due north. what is the angle between the net displacement vectors for the two birds?

Assume a 2D plane and both the birds start at origin. +X is east, -X is west, +Y is North, -Y is South. 
So, after travel position of bird1 is (-2.8, 2.7) and bird2 is (-2.7,2.8);
slope of bird1 is 2.8/2.7
slope of bird2 is 2.7/2/8
Now we have two solpes b1 and b2
formula = (b2-b1)/(1+b1*b2);