Just multiply the numbers as you would normally and then figure out the least residue (remainder) modulo 7:
I'm not sure whether in 6th grade you'll know about the meaning of "residue class"
Working modulo 7 means that whatever result you have, you work with the remainder of this result in the Euclidian division by 7.
Since 15=7x2+1, its remainder in the Euclidian division by 7 is 1.
Thus
Basically, if , then c divides a-b.