The rule is "whatever the input is, multiply it by 5, then subtract 2"
For example, if the input is 4 then 4*5 = 20 and 20-2 = 18
So in short, the input 4 leads to the output 18 as shown in the top row of the diagram.
The answer is the box on the bottom row, right hand side where it says "x 5" and "-2" in the two bubbles.