Step by step
The question
Question: solve y = x² − 4 and y = 3x.
We call them ① and ②.
We want the points (x, y) that make both true.
① is a parabola, and ② is a line.
Not both lines, so this is called a nonlinear system.
1 Put ② into ①
In ②, y is already alone: y = 3x.
Both ① and ② say what y is.
So the two right sides are equal.
2 Get 0 on one side
We take 3x away from both sides.
Careful!
The answers are points, not just x numbers.
After x, we must find each y.
Now we have x² − 3x − 4 = 0.
3 Factor
We need two numbers.
They multiply to −4, and they add to −3.
They multiply to a minus number, so one number is plus and one is minus.
They add to a minus number, so the bigger one is minus.
We list the pairs that make 4, smallest first.
We take the smaller from the bigger.
We need 3.
1 and 4 give 3.
Yes!
With the signs: 1 and −4.
So it is (x + 1)(x − 4) = 0.
4 One bracket is 0
Two numbers multiply to 0.
That only happens if one of them is 0.
So one bracket must be 0.
We make each bracket 0.
Two small equations.
In x + 1 = 0, we take away 1 on both sides.
In x − 4 = 0, we add 4 on both sides.
So x = −1 or x = 4.
5 Find y
Each x goes into the line ②: x = −1 gives y = −3.
And x = 4 gives y = 12.
So the two points are (−1, −3) and (4, 12).
6 Check
Let's check with a picture.
We draw both on a grid.
① is a parabola.
② is a line.
They meet at (−1, −3) and (4, 12): our answers.
Let's check each point in both equations.
First (−1, −3) in ①: the right side is −3, the same as y.
In ②: −3 again.
It works!
Now (4, 12) in ①: the right side is 12, the same as y.
In ②: 12 again.
It works!
Let's look back.
First: put ② into ①.
Then: get 0 on one side.
Then: factor.
Then: one bracket is 0.
Then: find y.
Last: check.