Expert: David Montiel - 8/24/2009

Question
If x = –1 and y = 2, what is the value of the expression 2x3 – 3xy ?
A.  8
B.  4
C.  –1
D.  –4
E.  –8


Answer
Hello Brooke,

Before I get to your question, I should let you know that my policy is not to give full
solutions to questions like these (homework), because I believe that it wouldn't really help you
learn. However I'll be glad to give you some hints to help you started.

OK, so you have 2x3 – 3xy. I am assuming that the "3" on the first term is "to the power of 3" right? Just so you know, when you are typing this, you can type "^" right in front of a number to indicate that it is an exponent. Specifically, in your example, you would write:

2x^3 – 3xy

you see?

Now all you need to do is "plug in" the numbers you are given and do the operations to compute the result. For instance, they tell you x = –1 and y = 2. So in the expression you were
given (2x^3 – 3xy), you just need to: a) replace every "x" you see with the value of "-1" and every "y" with the value "2", and b) do the math. Just remember the order you need to follow to do the operations - when there are no parentheses (like here) you do powers first, then multiplication, and finally, addition or substraction.

Hope this helps.

Cheers,
David