Look at the relationship between the second pair: 26 - 17 = 9, so the rule connecting the two numbers is 'add 9'. Apply the same rule to the first pair: 5 + 9 = 14. So the missing number is 14, option (3).
PRO TIP
Re-check your working by substituting the answer back into the original question.
Related Questions
A student scored 72, 68, x and 80 marks in four tests. If the mean marks of the four tests is 75, find the value of x.