Do you multiply or add when simplifying $λ$-expressions?

Question

The question is to simplify this λ-expression $(\lambda x. x) (\lambda x. \lambda y. y x) 8 (\lambda x. x + 1)$

My current thinking is as follows:

$(\lambda x. x) (\lambda x. \lambda y. y x) 8 (\lambda x. x + 1)$ = $(\lambda x. \lambda y. y x) 8 (\lambda x. x + 1)$

$(\lambda x. \lambda y. y x) 8 (\lambda x. x + 1)$ = $(\lambda y. y 8)(\lambda x. x + 1)$

$(\lambda y. y 8)(\lambda x. x + 1)$ = $(\lambda x. x + 1)(8)$

$(\lambda x. x + 1)(8)$ = $8+1$ = $9$

However, I am told the answer is 72. why would the 9 multiply the previous 8 after it's substituted in?