As hinted above, shift the origin by 3 to get a symmetric integral over 1 to 1. Call this integral 1.
Then replace x by x (and of course dx by dx) and see what you get as an equivalent integral. Call this integral 2.
Now add integral 1 and integral 2 to get an integral that is easier to do. Call this integral 3.
Now, since integral 1 and 2 are equal, they are each 1/2 of integral 3.
