Question

What is the equation of the line passing through `(21,4)` and `(18,-2)`?

Answer 1

`y=2x-38`

Explanation:

The equation of a straight line is `y = mx + c` . Where x is gradient and c is the y intercept.

`m = (deltay)/(deltax)`
(the symbol for delta is wrong. It is actually a triangle. Delta means "change in".)
So in our case:
`m = (4 - -2)/(21-18) = 6/3 = 2`
You can then substitute 2 into the equation: `y = 2x +c`

You can then find out what c substituting one of the co-ordinates in.
`y=2x+c` --> `4=21*2+c = 42 + c`
If you take away 42 from both sides `c = -38`

So the answer is `y=2x-38`