Can somebody explain to me how to do this

Basically, a positive hundreds-place multiplication is going to move your decimal to the right by 2 places. 

When you multiply the two together (which you can do in a calculator), you get an answer of 68.5. Again, this comes from moving the decimal in 0.685 two places to the right to receive your final answer. 

I hope I was able to help. Best of luck.