Ok so you have d ducks and c cows. We know d+c=24. Also, since ducks have 2 legs and cows have 4 legs, 2d+4c=66.
We now have a systems of equations to solve for a variable using elimination
2(d+c=24)
1(2d+4c=66)
2d+2c=24
2d+4c=66
-2c=-18
c=9
Now that we have the number of cows, we can easily find the number of ducks by plugging in 9 to the equation c+d=24
9+d=24
d=15
There are 9 cows and 15 ducks.