Answer:
Amount invested at 8% is $9000 and amount invested at 18% is $21000.
Step-by-step explanation:
Let amount invested at 8% be x.
Let amount invested at 18% be y.
We get the 1st equation as:
........(1)
We get the second equation as:

=> 
or getting rid of the decimal by multiplying by 100 on both sides.
........(2)
Multiplying (1) by 8 and subtracting from (2) we get
So, y = 21000
And 

So, x = 9000
Therefore, amount invested at 8% is $9000 and amount invested at 18% is $21000.
Slope (or gradient) is used to determine if lines are parallel, as parallel lines share the same gradient.
The gradient is used to determine whether they intersect, as you'll have to plug x and y values in to both of the line's equations, which involves having the line's gradients.
The gradient is used to see if points are collinear, as you need to determine whether the gradient between two points is equal to the gradient of another 2 points, and if so, the points are collinear.
Finally, the gradient is used to find the equation of the line, as we require to plug in the gradient in to equations such as (y - y1 = m(x - x1)) in order to find the equation of the line.
A is a long term-investment strategy
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This should help you out!