64-8+ 12 ×2 +9
apply the rule of BODMAS
and
that means you have to solve bracket first,
then of (as in 1/2 if 12)
then division 
then multiplication
and then addition,
and finally you can subtract
[BODMAS stands for bracket,  of,  division, multiplication,  addition,  subtraction]
Using BODMAS rule
64-8+ 12 ×2 +9
= 64 - 8 + 24 +9
= 97 - 8
= 89