SOLUTION: Can you please show the work on this for me? x=2 y=-4 the answer is -80 2x^2+3xy-4y^2 I can't get to -80 and not sure how to do the 3xy part? Thank you!

Question 548697: Can you please show the work on this for me?
x=2
y=-4
the answer is -80
2x^2+3xy-4y^2
I can't get to -80 and not sure how to do the 3xy part?
Thank you!
Answer by bucky(2189) (Show Source): You can put this solution on YOUR website!
You are asked to evaluate
.

.
when x = 2 and y = -4
.
Start by evaluating
.
When x = 2 then . This is equal to
.
Next evaluate
.
Substitute 2 for x and -4 for y to get:
.
. The 3*2 = 6 and then the 6*-4 = -24
.
Finally, evaluate when y = -4. Substitute -4 for y to get:
.
and this becomes
.
Now combine all three terms. Don't forget that this last one has a minus sign preceding it.
.

.
Remove the parentheses and then sum the terms on the right side to get the answer:
.

.
Combine the +8 and -24 to get -16. Then add the -16 to the -64 to get -80.
.
Hope this helps you to find where you were having trouble. In this case it could easily involve being confused by the signs.
.
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