When would I use an Index/Match function rather than vlookup?

The Index/Match function is the preferred way to look up a value in excel given it requires far less processing capability than vlookup. While vlookup uses the entire table array, index/match only considers the two columns you are both referencing and searching. This provides index/match users with a more versatile equation as the lookup function is not pertaining to a table at a whole and not compromised if for example, a column in the table must be deleted.

What is the domain and range of y=x^2-7?

First, let's start with the domain. The domain is defined as all x values which will yield a real y value. Since this is a quadratic function, x is being squared you must ask yourself, are there any values which if squared, will not produce a real number? Since the answer is no, the domain is (-∞,∞). Second, the range is defined as all possible values of y given the function of x. Since this is a quadratic equation, you must ask yourself, are there any values that x^2 cannot produce? Whether it be by looking at a graph, or simple guess and check, you will find that x^2 can never yield a negative number. Therefore, since the y intercept is -7, this will be the lowest possible y value, while the function will only continue to increase as the absolute value of x increases. Therefore the range is [-7,∞)

How do I find the y intercept of an equation?

Well if you picture a graph, the y-intercept is the value of y when the graph crosses the y axis. Further, the y axis represents x=0. So for example, lets say the given equation is y=5x^2+4. If we plug in 0 for x, we are left with 5(0)^2+4, which equals 4. Since y=4 when x=0, the y intercept is 4.