如图,抛物线\(y= \dfrac {1}{4}x^{2}+ \dfrac {1}{4}x+c\)与\(x\)轴的负半轴交于点\(A\),与\(y\)轴交于点\(B\),连结\(AB\),点\(C(6, \dfrac {15}{2})\)在抛物线上,直线\(AC\)与\(y\)轴交于点\(D\).
\((1)\)求\(c\)的值及直线\(AC\)的函数表达式;
\((2)\)点\(P\)在\(x\)轴正半轴上,点\(Q\)在\(y\)轴正半轴上,连结\(PQ\)与直线\(AC\)交于点\(M\),连结\(MO\)并延长交\(AB\)于点\(N\),若\(M\)为\(PQ\)的中点.
\(①\)求证:\(\triangle APM\)∽\(\triangle AON\);
\(②\)设点\(M\)的横坐标为\(m\),求\(AN\)的长\((\)用含\(m\)的代数式表示\()\).