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A line passes through (2,-1) and (5,y). Find the value of y if the slope of the line is 3.

I don't get it

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Robert J. | Certified High School AP Calculus and Physics TeacherCertified High School AP Calculus and Ph...
4.6 4.6 (13 lesson ratings) (13)

Use the slope definition as the ratio of the change of y to the change of x,

(y+1)/(5-2) = 3

Multiply both sides by 3,

y+1 = 9

Add -1 to both sides,

y = 8 <==Answer

Diego P. | Economics Major, Math Minor, Finance Equivalent. Quantitative Topics.Economics Major, Math Minor, Finance Equ...

I'll try breaking it down a different way.
If you find the formula for the line that these points are on, you can then plug any x-coordinate and receive it's y-coordinate.
Here, you're given two points that are on a single line.
The formula for a line is y = mx + b      
m is the slope , b is the y-intercept
You're given the slope m already. It's 3. And you have points for y and x.
Now you need to slove for b. 
Plug in your first full point (2,-1) => -1 = 2(3)+b
Solving for b = -1 - 2(3) = -7
Now we have our linear formula: y = 3x - 7
Now we can plug the x-coordinate of our next point (5,y) and we will get the y-coordinate
y = 3(5) - 7 = 15 - 7 = 8
y = 8  <= that's your answer