Area of the rectangle = Length x Width
We will calculate the percentage of uncertainties.
For a length: p 1 = ( 0.2 · 100 ) : 22.4 = 20 : 22.4 = 0.893
For a width: p 2 = ( 0.1 · 100 ) : 8.4 = 10 : 8.4 = 1.190
p 1 + p 2 = 0.893 + 1.190 = 2.083 %
A = 22.4 · 8.4 = 188.16 +/- 2.083 %
Uncertainty: 188.16 · 0.02083 = 3.92
A = 188.16 +/- 3.92 cm²