This is a 30-60-90 triangle. In a 30-60-90 triangle, the longer leg is √3 times longer than the shorter leg, and the hypotenuse is twice the length of the shorter leg.
Longer leg: shorter leg * √3
Hypotenuse: shorter leg * 2
In the diagram, the shorter leg would be 18/2 = 9. x is the longer leg, so x would be 9√3. y is the hypotenuse, so y would be 9*2 = 18.
x (longer leg) = 9 * √3
y (hypotenuse) = 9 * 2 = 18

If this is represented by a linear function, then we know that the amount of money decreases at an unchanging amount. Knowing this, and knowing that on day 8, she had 2400, we simply just add 2400 to 8 100's:
2400 + 8*100 = 2400 + 800
Carrie initially had $3200, or Option C.
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hope this helps!</span>