All straight lines can be written as `y = mx + c`, where the `m` and `c` are numbers.

The `m` gives you the slope, or **gradient** of the line.

Gradients can be positive or negative. A line rising from left to right is a positive gradient. A line falling from left to right gives a negative gradient.

*Up* and *along* can each be negative. Moving bottom to top is positive, and left to right is positive.

The `c` gives the intercept on the y-axis ie when `x = 0`.

What is the gradient and intercept of the line *y* = *-4x* - *3*?

From `y = mx + c`, `m` = -4 and `c` = -3.

Answer: The gradient is -4; the intercept is -3

Two points on a line are (0, 4) and (1, 7). What is the equation of the line?

It moves along 1 (from 0 to 1).

The line moves up 3 (from 4 to 7).

The gradient is `frac(3)(1)`: `m` = 3

Write the equation as `y = 3x + c`

When `x` = 0, `y = 4`

`4 = 3 xx 0 + c` therefore `c` = 4

`y = 3x + 4`

Answer: *y* = 3*x* + 4

Check out our iOS app: tons of questions to help you practice for your GCSE maths. Download free on the App Store (in-app purchases required).