Too easy!

We note that $x \oplus y = x \times y + x$ . Therefore, $8 \oplus 11 = 8 \times 11 + 8 = \boxed{96}$ .

There's a logic where you need previous solutions to get the answer 40 . But this logic fails if you change the statements order or remove any of them, giving different results for the same statements.

$x+y = x+y+$ (previous answer)

This way:

$8+11 = 8+11+21 = 40$

But, if you apply a logic where the results are the same, no matter the order of statements, or if there's no other statements at all, then you find the correct answer, 96 .

$x+y = x \times (y+1)$

This way:

$8+11 = 8 \times (11+1) = 96$

Plus, if you add more statements to complete the sequence (adding 1 to $x$ and $y$ on each statement):

$1+4 = 5$

$2+5 = 12$

$3+6 = 21$

$4+7 = 32$

$5+8 = 45$

$6+9 = 60$

$7+10 = 77$

$8+11 =$ ?

Then both logic works, but both will result 96 . Just remember that, only the second one works perfectly, as it independs from previous results or the existence of previous statements.