Info about coefficients of augmented matrix

In Lay is given the following problem:

Suppose the system below is consistent for all possible values of *f* and *g*. What can you say about the coefficients *c* and *d*? Justify your answer.

$\displaystyle x_{1} + 3x_{2} = f$

$\displaystyle cx_{1} + dx_{2} = g$

In the answers is the following:

The row reduction of $\displaystyle \begin{bmatrix} 1 & 3 & f \\ c & d & g \end{bmatrix}$ to

$\displaystyle \begin{bmatrix} 1 & 3 & f \\ 0 & d-3c & g-cf \end{bmatrix}$ shows that *d-3c* must be nonzero, since *f* and *g* are arbitrary. Otherwise for some choices of *f* and *g* the second row could correspond to an equation of the form *0=b*, where *b* is nonzero. Thus $\displaystyle d \neq 3c$.

This is just fine, what I don't understand however is the missing working between the initial matrix and the row reduced version. I can't even figure out which row operations to begin with.

Can someone please explain how this matix:

$\displaystyle \begin{bmatrix} 1 & 3 & f \\ c & d & g \end{bmatrix}$

Row reduces to this:

$\displaystyle \begin{bmatrix} 1 & 3 & f \\ 0 & d-3c & g-cf \end{bmatrix}$

ty

Re: Info about coefficients of augmented matrix

The second row is replaced by the sum of the second row and the product of the first row times -c.

Re: Info about coefficients of augmented matrix

That makes sense in such a simple way that it leaves me feeling very stupid.