Question

if p(x)=x+3,then what is the value of p(x)+p(-x)

Answer 1

p(x) = x+3
p(-x) = -x+3
p(x)+p(-x) = (x+3)+(-x+3)
               = x+3-x+3
p(x)+p(-x) = 6

Answer 2

P(X) =  X + 3 
This means that what ever is in the parenthesis on LHS is what X is on RHS.  Replace X on RHS with the expression within parenthesis on LHS.

P( 3 ab + 4 g - 8 k ) will be  =  (3 ab + 4 g - 8k)  + 3
============================================
P(X)  =  X  + 3
P(-X) =  (-X)  + 3    =  3 - X

P(X) + P(-X)     =  X + 3 + 3 - X = 6