Answer:
7

Explanation:
the altitude creates two congruent 30-60-90° triangles
the ratio of the sides in a 30-60-90°, respectively, are 1 :
: 2
therefore we can set up a proportion to find the side length:
let 'x' = side length of equilateral Δ
/ 1 = 21 / x
cross-multiply:
x = 21
x = 21 /

rationalize the denominator by multiplying numerator and denominator by

21
/
·
= 21
/ 3 or 7
