Hook up Sanyo tv model No.DP50741, to an Roku model No.W16-005N1A with composite cable adaptor

That Roku model number seems to be the part number of the power supply not the box.
Your TV has 3 HDMI inputs so why not use one. If you already use all three the best thing to do is get an HDMI input selector to expand the number of inputs. This way you get the best picture.
If you have to use AV then the best way is to get a Roku Express + that has AV outputs built in.
If you have to use a Roku you have you will need an active HDMI to component video plus audio converter. A passive cable won't work.
Connect the HDMI output of the Roku to the converter input. Connect the component video plus audio cables (5 RCA) from the converter out to the TV in. Using a single yellow video instead of the three red green blue video connection will degrade the picture quality.