Answer:
28
Explanation:
notice that when x=21 thats 3x7=21
then when y=49 thats 7x7
so when y=56 that's 8x7
notice there is a patter of 7 being used? there are probably a lot of other patterns that would also work.. but. if your class is working on multiplying by 7 , then this is probably the right pattern.
from the above pattern, then,
x=4x7 when y=56
x=28