SOLUTION:
Let whole number = x
x + 2x^2 = 21
2x^2 + x - 21 = 0
2x^2 + 7x - 6x - 21 = 0
x ( 2x + 7 ) - 3 ( 2x + 7 ) = 0
( 2x + 7 ) ( x - 3 ) = 0
2x + 7 = 0
2x = - 7
x = - 7 / 2
OR
x - 3 = 0
x = 3
ANSWER:
Therefore, as 3 is a whole number while - 7 / 2 isn't a whole number, the whole number must be 3.
Hope this helps! :)
Have a lovely day! <3