To factor an expression, you need to analyze it and find a common variable or expression on the expression given. In this expression, you can observe that the expression x^2 – 7 is common to the two terms. You need to factor it out from the both of them.
4x (x^2 – 7) + 3 (x^2 – 7)
(x^2 – 7) (4x + 3)
This is already the complete factor of the expression.