Dennis can load the car in 28 min. if andy him it takes 21 min to load the car how long will it take andy alone?

Let the load car = y , and Andy will take time x (min.) to load the car
So,
    The rate of Andy to load the car = y/x

And we given that: Dennis load the car in 28 min.
So,
    The rate of Dennis to load the car = y/28

If both are loading the car in time = t = 21 min
then:
         y = (y/x) t + (y/28) t     ⇒ divide all equation by y and substitute with t
∴      1 = (21/x) + (21/28)     ⇒ multiply all equation by 28x
        28x = 21*28 + 21x
        28x - 21 x = 588
        7x = 588
∴      x = 84 min.

Which mean Andy will take 84 min. to load the car alone.