Let s and l be the price of small and large dishes. The first equation is
[tex]3s+5l = 28.20[/tex]
While the second equation is
[tex]4s+3l=23.30[/tex]
(I assume that you meant "4 small dishes and 3 large dishes")
If we multiply the first equation by 4 and the second by 3 we have
[tex]\begin{cases}12s+20l=112.8\\12s+9l=69.9\end{cases}[/tex]
Subtract the two equation and you have
[tex]11l = 42.9 \iff l = 3.9[/tex]
Substitute this value in one of the equation to derive
[tex]3s+5\cdot 3.9 = 28.20 \iff s=2.9[/tex]
So, one small dish costs 2.9 and one large dish costs 3.9. This means that 2 small and 4 large dishes cost
[tex]2.9\cdot 2 + 3.9 \cdot 4=21.4[/tex]
