Vector Calculus: Surface Integrals

I really don't even know where to start on this problem.

I found this formula in the book:

The surface integral of F over S is:

Dbl Integral (**F** dot d**S)** = Dbl Integral (**F **dot **n** d*S*)

This is also called the flux of **F **across *S*.

and:

Dbl Integral (**F** dot d**S**) = Dbl Integral(**F **dot (**r** sub u X **r **sub v) dA

Here is the problem:

Evaluate: Dbl Integral (**A** dot **n** d*S*) over *S *in each of the following cases:

(a) **A** = y**i **+ 2x**j** - z**k** and *S* is the surface of the plane 2x + y = 6 in the first octant cut off by the plane z = 4.

(b) **A **= (x + y^2)**i **- 2x**j **+ 2yz**k** and *S *is the surface of the plane

2x + y +2z = 6 in the first octant.

The answers are (a) 108. (b) 81.

Thanks a lot! It'd be great if someone could help me out on here by morning or I'm going to have to go talk to a TA for help.