P(n) = 3^n - 1 is the function they give you, where n is a natural number, that means that is can be 1, 2, 3, 4...
The first part of the equation is 3 ^n, that is the 3 multiplied by itself n times... for example :
When n = 1, P = 3^1 - 1 = 3 - 1 = 2 which is muliple of 2
When n= 2, P = 3^2 -1 = 9 - 1 = 8 which is also multiple of 2
When n = 3, P = 3^3 - 1 = 27 - 1 = 26 which also is a multiple of 2
3^n will always be an odd number, so 3^n - 1 will always be an even number
3 is an odd number
an odd number multiplied by an even number will always be even
3^n means multiply 3 by 3 n times, so it always be multiplied by 3