tolony bantu ya :( terimakasih

gradien (m) a = 2
gradien (m) b = -1/2
gradien (m) c = 2 (karena sejajar garis a)

Koordinat garis b = (0,2) , (4,0)
Koordinat titik c = (4,0)

PGL garis c :
y-y1 = m(x-x1)
y - 0 = 2 (x-4)
   y   = 2x - 8 (dipindah ke ruas kiri)
y - 2x + 8 = 0 (C)