-2 * -21=42
-2-21= -23 NOPE
-3*-14 = 42
-3-14= 17 NOPE
-7*-6= 42
-7-6 = -13 YES!!!
The answer is -7 and -6.
You can know that the number is a multiple of another number if a number is divisible by another number. Then it can be a multiple of that number. :)