Question

if 14 men can cultivate 42 acres of land in 18 weeks how many week will 21 men working at the same rate take to cultivate 56 acres of land

Answer 1

42 acres requires 14*18 man-weeks of work
so, 56 acres requires 56/42 * 14*18 man-weeks of work
Now divide that by 21 men and you get 16 weeks

Or, note that you have
3/2 the men, so 2/3 the time
4/3 times the area, so 4/3 times the time
18 * 2/3 * 4/3 = 16

or, as a word problem,
The time it takes to clear a field varies directly with the area, and inversely with the number of men. So,

t = ka/m

That is, tm/a = k, a constant
So, you want t such that
t*21/56 = 18*14/42
t = 16

Answer 2

or
14 men can cultivate 42 acres of land in 18 weeks
1 man can cultivate 42/14 or 3 acres of land in 18 weeks
1 man can cultivate 1 acre of land in 6 weeks
21 men can cultivate 1 acre of land in 6/21 or 2/7 weeks
21 men can cultivate 56 acres of land in 2/7(56) or 16 weeks