解如下图所示
解:因为∠ABC+∠ACD+21°+∠DAC=180°，∠ABC=∠ACB所以2∠ABC+∠DAC=159°因为∠ADC=∠DAC,∠ADC又是三角形ABD的外角所以∠DAC=∠ABC+21°因为2∠ABC+∠DAC=159°，∠DAC=.∠ABC+21°所以2∠ABC+∠ABC+21°=159°所以∠ABC=46°
