Hello from MrBillDoesMath! 
 
Answer: 
 0.5 x^4 + 12.5
Discussion: 
 Only the constant terms are "like" and can be combined. Nothing combines with x^4.
0.5 ( x^4 - 9) + 17 =
0.5 x^4 - 0.5 (9) + 17 = as 0.5(9) = 4.5
0.5x^4 - 4.5 + 17 =
0.5 x^4 + 12.5
 
Thank you, 
 
MrB