First of all, the square root of 28= sq.root(7*4).....so it is also equal to 2 square root 7....so... 3 root 28 = 6 root 7.

so, 5 square root 7 + 6 square root 7 = 11 square root 7.

I dont know if my math is correct, but you just have to simplify your square roots.

Sorry for the sloppy math....