Which of these expressions represent the sum of 3 and n?

Don't really need to post the options to know what the expression(s) should look like. 

"Sum of" tells you its addition and because it's addition the order of the two terms (the 3 and the N) don't matter. 

Your correct expressions are:

3 + n
or 
n + 3

1. isolate the variable, in this case, d. subtract 35.5 on both sides of the equation.2. 52 subtracted by 35.5 is 16.53. therefore your answer would be d= 16.5this is the basics of pre algebra, isolating variable through adding, subtracting, multiplying, and dividing, if you have any other questions to ask just comment!