Answer:
17
Step-by-step explanation:
you need to factorize the value of a×b since you know that they are whole numbers which means there is no fraction or irrational or negative numbers
you will find that 42=21×2=7×6=14×3=21×1
then you begin to find the difference of the numbers
21-2=9
7-6=1
14-3=11
21-1=20
then you will notice that the third one is right as a=14,b=3
then you take their sum