The sequence 4;9;x;37 is a quadratic sequence

Question

1.1 calculate x

Answer 1

In a quadratic sequence, the 2nd differences are constant
terms of 1st sequence: 9-4, x-9, 37 - x
terms of 2nd sequence:
x-9 - 5, 37-x - (x-9)
or : x - 14, 46 - 2x

then x - 14 = 46 - 2x
3x = 60
x = 20