[tex]\left\{\begin{matrix}
2a + 4p = 16 \: | \times ( - 2) \\ 4a + 20p = 68\end{matrix}\right.[/tex]
[tex]\left\{\begin{matrix}
- 4a - 8p = - 32\\
4a + 20p = 68\end{matrix}\right.[/tex]
[tex] - 4a + 4a - 8p + 20p = - 32 + 68[/tex]
[tex]12p = 36[/tex]
[tex]p = \frac{36}{12} [/tex]
[tex]p = 3[/tex]
[tex]2a + 4p = 16[/tex]
[tex]2a + 4 \times 3 = 16[/tex]
[tex]2a + 12 = 16[/tex]
[tex]2a = 16 - 12[/tex]
[tex]2a = 4[/tex]
[tex]a = \frac{4}{2} [/tex]
[tex]a = 2[/tex]