2+7+(5+7//2x6-8)

If we have  2+7+(5+7/2*6-8)    ....evaluate inside the parentheses......and remembering that we perform multiplication/division as we encounter them from left to right, we have.....

2 + 7 + (5 + 3.5 x 6 - 8) =

9 + (5 + 21 - 8) =

9 + (26 - 8)  =

9 + 18  =

27

That is 27.......you mistyped it though

2+7+(5+7//2x6-8)

9(12//12-8)

9(12//4)

108//36

108 divided by 36 equals 3.