解：因?yàn)榧淄瑢W(xué)看錯(cuò)?$a$?的符號(hào)，
所以把?$x = 3，y = 2$?代入?$x + cy = 4$?得：
?$3 + 2c = 4，$?解得?$c=\frac {1}{2}。$?
?$ $?且由方程組的第一個(gè)方程可得?$-3a + 2b = 6。$?
?$ $?因?yàn)橐彝瑢W(xué)看漏?$c，$?
所以把?$x = 6，y = - 2$?代入?$ax + by = 6$?得
?$6a-2b = 6。$?
?$ $?聯(lián)立?$\begin {cases}-3a + 2b = 6\\6a - 2b = 6\end {cases}，$?
兩式相加得：?$3a = 12，$?解得?$a = 4。$?
?$ $?把?$a = 4$?代入?$-3a + 2b = 6$?得：?$-12 + 2b = 6，$?
解得?$b = 9。$?
?$ $?所以?$2a - b + 4c=2×4-9 + 4×\frac {1}{2}=8 - 9 + 2 = 1。$?
