$$\eqalign{
& a)\,\,Xet\,\,{\Delta _v}ABH:\,\,\widehat {ABH} + \widehat {BAH} = {90^0} \cr
& Lai\,\,co:\,\,\widehat {BAH} + \widehat {HAC} = \widehat {BAC} = {90^0} \cr
& \Rightarrow \widehat {ABH} = \widehat {HAC} \cr
& hay\,\,\widehat {ABC} = \widehat {HAC} \cr
& b)\,\,Ta\,\,co:\,\,\widehat {BAM} + \widehat {MAC} = {90^0} \cr
& \Rightarrow \widehat {BAM} + {70^0} = {90^0} \cr
& \Rightarrow \widehat {BAM} = {20^0} \cr
& \Rightarrow \widehat {BAM} = \widehat {MAH} = {20^0} \cr
& Xet\,\,{\Delta _v}AMH:\,\,\widehat {MAH} + \widehat {AMH} = {90^0} \cr
& \Rightarrow {20^0} + \widehat {AMH} = {90^0} \Rightarrow \widehat {AMH} = {70^0} \cr
& Vay\,\,\widehat {AMC} = {70^0} \cr} $$