Для начала найдем значение cos(a) по формуле:

sin^2(a) + cos^2(a) = 1

(√5/21)^2 + cos^2(a) = 1
5/441 + cos^2(a) = 1
cos^2(a) = 1 - 5/441
cos^2(a) = 436/441
cos(a) = ±√(436/441)
cos(a) = ±√436 / √441
cos(a) = ± √(4x109) / 21
cos(a) = ± 2√109 / 21

Так как pi/2 ≤ a ≤ pi, cos(a) < 0
cos(a) = -2√109 / 21

Теперь найдем √(21)cos(a):

√21cos(a) = √21 * (-2√109 / 21)
√21cos(a) = -2√109

Итак, √21*cos(a) = -2√109.