.

Do $\displaystyle R_2 + 2R_1$ and $\displaystyle R_3 - 2R_1$.

$\displaystyle A + 2B + 4C = D$

$\displaystyle 4B + 2C = 2D$

$\displaystyle -3B - 5C = -D$

Do $\displaystyle \frac{R_2}{4}$

$\displaystyle A + 2B + 4C = D$

$\displaystyle B + \frac{1}{2}C = \frac{1}{2}D$

$\displaystyle -3B - 5C = -D$

Do $\displaystyle R_3 + 3R_2$

$\displaystyle A + 2B + 4C = D$

$\displaystyle B + \frac{1}{2}C = \frac{1}{2}D$

$\displaystyle -\frac{7}{3}C = \frac{1}{2}D$

I take it now you can solve for $\displaystyle A, B, C$?