Agar garis-garis 2x-y-1=0, 4x-y-5=0, dan ax-7-y=0 melalui satu titik, maka nilai a?

jawab

2x - y = 1
4x -y = 5 ...(-)
-2x = -4
x = 2  

2x - y = 1
4 - y = 1
y = 3

ax - 7 -y= 0
a(2) - 7 - 3 = 0
2a - 10 = 0
a = 5