Need help understanding each step, please

A 4-year project has the following cash flows: I=$4;C1=+$9;C2=$4;C3=+$7;C4=$3. Use the triangle method to compute the bottom row; i.e., the row below the horizontal line. What is the sum of all the numbers in the bottom row? Give the answer as an integer. QUESTION 9 Exactly how many IRRs does a 18-year investment project have? Give the answer as an integer number