Lucy and lily are going to do their homework this evening .(

用often改为：Lucy and Lily often do their homework in the evening.
Lucy and lily are going to do their homework this evening.意思是：Lucy和Lily今晚将要做他们的作业.那么用often,也就是把将来时改为一般现在时,应该把这句话的意思改为：Lucy 和Lily经常在晚上做作业.也就是上面的答案.this evening：今天晚上——>in the evening：在晚上

Lucy and Lily often do their homeworks in the evening.

Lucy and lily often do their homework in evenings.