Question 735082
let the numbers be 
a-d, a , a+d

sum = 21

a-d+a+a+d=21

3a=21
a=7

(a-d)(a)(a+d)=280

(a^2-d^2)*a=280

(49-d^2)*7=280

49-d^2= 280/7
49-d^2= 40
d^2=49-40
d^2=9
d= +/-3

(a-d),a,(a+d)

if d=3

4,7,10

if a=-3
10,7,4
(Academic)