求助一道数学题

84. In the x-y plane, what is the y-intercept of the line l?
1). The slope of the line l is 3 times its y intercept.
2). The x-intercept of line l is -1/3
Answer:
y=kx+b
1). k=3b
2). -b/k=-1/3 => k=3b

哪位可以帮忙解释下条件２，为会什么与The x-intercept of line l is -1/3
可以写成Ｋ＝３Ｂ　我不明白．

x-intercept,就是当y=0时,x的值

根据上面的式子 y=kx+b, 令y=0,

则有 kx+b=0 => x=-b/k=-1/3