To solve the quadratic equation \(3x^2 + 2x - 8 = 0\), Martin first recognizes that he can group the terms in a way that allows for factoring.
From the structure of the equation, he is likely trying to factor by grouping. The format \(3x(x+2) - 4(x+2) = 0\) indicates that he correctly factored out a common term from parts of the equation. The common term here is \(x + 2\).
Thus, the correct missing step would involve factoring out the common factor, which leaves us with:
\[ (3x - 4)(x + 2) = 0 \]
So, given the choices provided in the question, the line of equation that Martin likely wrote before the one you shared is:
\[ 3x(x + 2) - 4(x + 2) = 0 \]
This corresponds to the first option:
3x(x + 2) - 4(x + 2) = 0
This line indicates the correct grouping and shows the factorization step required to progress in solving the quadratic equation.