Question

What is the equation of a line through the points (0,9) and (3,0)

Answer 1

The slope of the line is 9-0 / 0-3 = -3

the y intercept is at y= 9

y = mx + b 

so here we have 


y = -3x + 9 answer

Answer 2

Y = kx + b

(0,9)    x = 0;  y = 9
y = kx + b
9 = k*0 + b
9 = b

(3,0)     x = 3; y = 0
y = kx + 9
0 = k*3 + 9
-9 = 3k
k = -3

The equation of a line   y = -3x + 9