What is the value of the expression $2x^2 + 3xy – 4y^2$ when $x = 2$ and $y = - 4$? What is the value of the expression $2x^2 + 3xy – 4y^2$ when $x = 2$ and $y = - 4$?
I'm not good at algebra so please explain in easy to understand steps.
Thanks
 A: $2(2*2) + (3*2*-4) - (4*-4*-4) = -80$
$x^2 = x*x$  -> example: $2^2=2*2=4$
$xy = x*y$
$y^2 = y*y$ -> see first example
first term, $2x^2$ as shown above can be written as 2*x*x
second term, $3xy$ can be written as 3*x*y
lastly, $4y^2$, 4*y*y
you can write brackets around each term, it helps with negatives
$(2*x*x) + (3*x*y) – (4*y*y)$
plugging in the givin x and y values gives us
$(2*2*2) + (3*2*-4) - (4*-4*-4)$
The first one is straight forward, 2*2*2 = 8
however the -4 might throw people off
the second term, $(3*2*-4)$ the first two numbers are positive so we can just multiply normally
This gives (6*-4), but when multiplying by a negative number by a positive number the product will always be negative $(6*4)=24$ so $(6*-4)=-24$
In the third term we have a double negative
$(4*-4*-4)$ when multiplying a negative number by a negative number the product will always be positive
(4*-4*-4) -> (4*16) -> 64
together our equation looks like this
$(8) + (-24) - (64)$ to compute we need to use the properties listed above, you can think of it like this all terms have a 1* in front of them, 1 multiplied by anything will give you the same number.
$1*(8) + 1*(-24) - 1*(64)$
$1*(8)$ -> $8$, positive*positive=positive
$1*(-24)$ -> $-24$, positive*negative=negative
$-1*(64)$ -> $-64$, negative*positive=negative
which gives us, $8 - 24 - 64 = 8 - 88 = -88 + 8 = -80$
A: To find this value, everywhere you see an $x$, replace it with a $2$, and replace every $y$ with a $-4$, so the new expression looks like 
$$
2(2)^2+3\cdot(2)\cdot(-4)-4(-4)^2
$$
Then just do the operations shown above to find what the value is.
A: Subbing in $x=2$ and $y=-4$ into the equation gives: 
$2(2^2) +3(2)(-4)-4((-4)^2)=2(4) + (-24) -4(16)=8-24-64=8-88=-80$
A: $2x^2 + 3xy - 4y^2$ when $x = 2$ and $y = -4$, 
$2(2)^2 + 3(2)(-4) - 4(-4)^2 = 2 *4 - 24 - 4*16 = 8 - 24 - 64 = -80$
