well fist you would take the two ends and add them so 21+21= 42cm² then find the other four sides two would equal 24 cm² and the other two would equal 56cm² this is because the two numbers that multiply to 21 and add to 10 (it is 10 not 20 because there are four sides and it must be divided by two to find just two numbers) are 3 and 7. so the surface area ends up being 21+21+56+56+24+24 which equals 202cm²
