1/2×2√21×4√7= .... pake cara ya

salah maaf.............

1/2 x 2 √21 x 4 √7
= 1/2 x 2 √3 √7 x 4 √7
= 1/2 x 2 x 4 x √7 x √7 x √3
= 4 x 7 x √3
= 28 √3

semoga membantu
maaf yah kalo salah