We can start off by simplifying this expression:
3(x - 4) + 4(7 - 2x^2 + 5x) - 8(2x - 1)
3x - 12 + 28 - 8x^2 + 20x - 16x - 8
3x + 16 - 8x^2 + 4x - 8
7x + 8 - 8x^2
Coefficient of x is the number in front of x.
Which means the coefficient of x in 7x is 7.
