Question

K^2= 4 (mod 7) or K^2 has a remainder of 4 when divided by 7

k^3 = 1 (mod 7) or K^3 has a remainder of 1 when divided by 7
3k = ? (mod 21) or 3K has a remainder of ? when divided by 21

Answer 1

Since 3/21 = 1/7
it's really..

k = ? (mod 7)

looking at the first given
k² = 4 (mod 7)
(k*k)/7 = dividend + remainder (2*2)

remainder 2 for each k
k = 9

When there are 3 as in ;
(k*k*k)/7 = dividend + remainder (1)
2*2*2 =8 and another 7 divides out leaving remainder 1.

So the question mark or remainder for 3k = ? (mod 21) is 2

Hope this helps.