if you look at the question we have equations as

2+9=711

14+6=820;

17+11=628

then 12+3=??

from Equation 1 

2+9=711

2+9=(2-9)(2+9)

= 711

here we used a formula called a+b=(a-b)(a+b)

similarly for remaining equations

Equation:2

14+6=(14-6)(14+6)=820

Equation:3

17+11=(17-11)(17+11)=628

now we have 12+3=??

12+3=(12-3)(12+3)=915

12+3=915 is the correct answer

Addition of 1/2 with the previous number.

1, 1+1/2=3/2,

3/2+1/2=2/1,

2/1+1/2=5/2, and lastly

5/2+1/2= 3/1

Correct answer: 3/1