I've got the following review problem in my micro-econ course:

y = f(x) = 5x^2 - 4x

(a) Find delta-y/delta-x as a function of x and delta-x.

(b) Find dy/dx.

(b)'s just the derivative and I've confirmed the answer in the book. However, I cannot work out how one arrives at this solution for (a):10x +(5delta-x)- 4

Where does the5delta-xcome from?

The text gives this form:

delta-y/delta-x = [f(x - delta-x) - f(x)] / delta-x

Using that, I got 5delta-x - 6

Any help would be greatly appreciated. Thanks in advance.

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