Question

A line passes through the points (1, –6) and (4, 3).

What is the y-intercept of this line?
–9
–3
3
9

Answer 1

y = - 9

Explanation:

The equation of a line in slope- intercept form is

y = mx + c ( m is the slope and c the y- intercept )

Calculate m using the slope formula

m = (y₂ - y₁ ) / (x₂ - x₁ )

with (x₁, y₁ ) = (1, - 6) and (x₂, y₂ ) = (4, 3)

m =
`(3+6)/(4-1)` =
`(9)/(3)` = 3

y = 3x + c ← is the partial equation

To find c substitute any of the 2 points into the partial equation

Using (4, 3), then

3 = 12 + c ⇒ c = 3 - 12 = - 9, hence

y- intercept c = - 9 ⇒ (0, - 9 )

Answer 2

=-9 (the first choice)

Explanation:

To find the y-intercept we must first find the equation of the line in the form y=mx + c where m is the gradient and c is the y- intercept.

m=Δy/Δx

=(3--6)/(4-1)

=9/3

=3

Let us write the equation using any of the given points, say (4,3)

(y-3)/(x-4)=3

y-3=3(x-4)

y-3=3x-12

y=3x-12+3

y=3x-9

Using the format y=mx+c, the y-intercept is -9