Question

find three numbers in AP so that their sum is 21 and the product of the last two is 84

Answer 1

let the terms be a-d,a , a+d
$(a - d) + a + (a + d) = 21 \\ 3a = 21 \\ a = 7 \\ a(a + d) = 84 \\ 7(7 + d) = 84 \\ 7 + d = 12 \\ d = 5 \\ a - d = 2 \\ a = 7 \\ a + d = 12$
they are 2,7,12

Answer 2

These 3 numbers are 2,7,12