Mohon Dibantu No 21 PAKAI CARA SUPAYA MUDAH DIPAHAMI

A + ( 3a + 20 )° = 90
A + ( 11a - 10 ) ° = 180

dieliminasi jadi :
( 3a + 20 ) - ( 11a - 10 ) = - 90
-8a + 30 = - 90
-8a = 120
a = -120 / - 8 = 15

dimasukkan kembali kepersamaan :
A + ( 3a + 20 ) = 90
A + ( 45 + 20 ) = 90
A = 90 - 65
   = 25° ( A )

semoga membantu....