vb calculation formula problem

my problems goes like this:

1. Input the value of T:3

Input the value of S: 6

Then when i click the compute button the formula will compute like this

T^S - [T(T-1)^S] + [T(T-2)^S] = A(answer)

for example if i input T=3 and S=6 the computation will goes like this...

3^6 - [3 (2)^6 ] + [3 (1)^6] =

729 - 192 + 3 = 540

^ = raised to the power

problem: a.) how can i decrement the value of T depending on the values i input

b.) how can i make an alternate conditional statement of the - and + operation.. because the formula will be

using an alternate operation - + - + and so on...

visual basic please thnx (Itwasntme)

hope someone can help me thank u very much(Rock)