m=-4, (-1,5)?

Question

Find an equation of the line having the specified slope and containing the indicated point. Write your final answer as a linear function in slop-intercept form. Then graph the line.

I don't need to know how to graph....

I have y-5=4(x+1)
y=4x-1
f(x) = 4x-1..... I got this one marked wrong on my homework and need to know why???

Answer 1

First of all the slope is -4 not 4, so without even doing any calculations I can see that much is wrong.

Other than that, you did approach the problem correctly.

y-5 = -4(x+1)

y-5 = -4x -4

y = -4x + 1

Be careful when working with negatives to not drop the sign, as it will mess you up! (I did that a lot when I was in school too!)

Answer 2

OH... you tripped yourself up by putting the y coordinate into the problem instead of replcing the y. You know the general equation for a line...

y=mx+b

You know that the slope is going to replace the m.
y=-4x+b

now... you know your y coordinate is -1 and your x coordinate is 5... so replace those into your equation and solve for b..

5=-4(-1)+b

So now...
5=4+b
b=1
So your equation is
y=-4x+1

The way you did it, when you solved for the equation, you got your negatives mixed up.

Answer 3

You might have tried to do it too quickly.

y - 5 = -4(x + 1) For some reason you used a positive slope.
y - 5 = -4x -4
y = -4x + 1