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Slope = rise / run   for the two points given

          =(8-0)/(7-9) = -4

so the equation of the line looks like   y = -4x + b      to find 'b' sub in one of the given points

                                                            0 = -4(9) + b     b = 36

y = -4x + 36       y intercept occurs when x = 0       y = 36              Y-intercept + slope = 36 -4 = 32

Find the sum of the slope and $y$-intercept of the line through the points $(7,8)$ and $(9,0)$.

First, make 

(7) the X1 and (8) the Y1

Then use the next point 

(9) as the X2 value and the (0) the Y2

 Then use the equation 

M=Y2-Y1

-------------

    X2-X1

Plug in The coordinate points and it should look like this

M= 0 - 8

-------------

     9 - 7

Then solve and you get 

M= 8

--------

     2

If it can be simplified then simplify it.

Then you will get the value of M= 4.

Then you use the formula 

Y=Mx+B

So you already know your slope, now you just have to solve for B.

So chose a point from your graph and plug-in.

For example the point (7,8)

8 is the Y-value

7 is the X-Value

Plug-in = 

8= 4(7)+b

8= 28 +b

Minus 28 from 8 

and you get

20= B

Your Final Answer is 

Y=4X+20