Question 658081
The original photo has an area of:

A=length*width
A=6 in * 8 in = 48 in^2

The desired area is half this or 24 in^2.

The product of the new length and width must equal 24 in^2. 
If you reduce both sides by 2 inches, the new photo is 4 in x 6 in. Both sides are reduced by 2 inches and the resultant area is halved.