Tolong bantu nomer 21 ya

3x-40+x+84=180°
4x+44=180°
4x+44-44=180-44
4x=136
X=34°
Sudut C=3x-40=62°
Jumlah sudut ∆ABC=180°
Sudut A+B+C=180°
Berarti kalo sudut B yg dicari
=180-sudutA-sudutC
=180-54-62=64°

(3x - 40) + (x + 84) = 180
3x - 40 + x + 84 = 180
4x + 44 = 180
4(x + 11) = 180
x + 11 = 45
x = 34

3x - 40 = 3(34) - 40 = 102 - 40 = 62

62 + 54 + sudut ABC = 180
116 + ABC = 180
ABC = 64

*****(atau)*****

(3x - 40) + (x + 84) = (3x - 40) + 54 + ABC
x + 84 = 54 + ABC
34 + 84 = 54 + ABC
118 = 54 + ABC
64 = ABC