Expanding the left side of the equation:

(7x + 1)(3x - 1) - 21x^2
= 7x 3x + 7x (-1) + 1 3x + 1 (-1) - 21x^2
= 21x^2 - 7x + 3x - 1 - 21x^2
= -4x - 1

Therefore, the equation simplifies to:

-4x - 1 = 3

Adding 1 to both sides:

-4x = 4

Dividing by -4:

x = -1

So, the solution to the equation is x = -1.