How to work it out and the answer for this question

If you use an x value of 3.5 you get that the right side is 49.875 which could feasibly be rounded to 50.  If you use an x value of 3.6 you get that the right side is 53.856 which is way too high.  To show your "working", just sub in 3.5 for each x: (3.5)^3 + 2(3.5)=50 and you'll get 49.875=50 (which it doesn't obviously, but it's as close as we are going to come since the directions told us to keep our number to one decimal place.)