First, start off by listing a few of the numbers that follow the first three clues to see if you can narrow it down. 7,111,111 7,333,333 7,555,555 etc. Then, start adding up digits to see if you're getting close. 7+1+1+1+1+1+1 = 13 7+3+3+3+3+3+3 = 25 7+5+5+5+5+5+5 = 37 Since 7,555,555 is too high, we step it down to 7,555,333 7+5+5+5+3+3+3 = 31 7,555,333 will work as an answer, as well as 7,333,555, since it's the same amount when the digits are added together.
D is a function because it passes the vertical line test, meaning that you can draw a vertical line anywhere through the graph and it will only it the lines one time.
