To simplify these expressions, we can rationalize the denominators.

1. To rationalize the denominator of the first expression, we multiply both the numerator and denominator by the conjugate of the denominator, which is (x + 3):

(2021 / √(x - 3)) * (√(x - 3) / √(x - 3)) = 2021√(x - 3) / (x - 3)

So the simplified expression is 2021√(x - 3) / (x - 3).

2. To rationalize the denominator of the second expression, we multiply both the numerator and denominator by the conjugate of the denominator, which is (x - 2021):

(2020 / √(x - 2021)) * (√(x - 2021) / √(x - 2021)) = 2020√(x - 2021) / (x - 2021)

So the simplified expression is 2020√(x - 2021) / (x - 2021).

3. To rationalize the denominator of the third expression, we multiply both the numerator and denominator by the conjugate of the denominator, which is (√(x - 4)):

(-15 / √(x - 4)) * (√(x - 4) / √(x - 4)) = -15√(x - 4) / (x - 4)

So the simplified expression is -15√(x - 4) / (x - 4).