Let's denote the length of the shorter parallel side as 'a' (5 cm), the length of the longer parallel side as 'b' (9 cm), and the distance between the parallel sides (height) as 'h'. The formula for the area of a trapezium is:

Area = (1/2) × (sum of parallel sides) × height

Given the area (A) as 21 cm², the formula becomes:

21 = (1/2) × (5 + 9) × h

Simplify the equation:

21 = (1/2) × 14 × h

Now solve for 'h':

21 = 7h

Divide both sides by 7:

h = 3 cm

So, the distance between the parallel sides is 3 cm.