Determine whether the equation defines y as a function of x. (see example 9.) 5x + 4y = 20

Strictly speaking, the equation does NOT define y as a function of x. It defines a relationship that x and y must satisfy. It can be solved for y so that it does define y as a function of x:

... y = (20 - 5x) / 4

However, if we take as a working definition of a function the notion that it is an equation that yields exactly one value of y for each value of x put into it, then we probably have to say the equation DOES define y as a function of x.

Your choice of answers likely depends on what you see in Example 9.