It would be correct if you'd type it correctly,
putting parentheses in the right place, like this:

11^7 x 11^8 ÷ 11^20
= 11^(7 + 8 - 20)
=11^(-5)
=1/11^5

What that means is this:






When you have to type mathematical expressions all on one 
line, you have to put numerators, denominators and exponents
within parenthese to show where they start and where they
end.  Parenthenses are not necessary only when a numerator,
denominator or exponent consists of only one letter or one
number.  Any other time parentheses are necessary.

Edwin