Remember, the slope-intercept equation is y=mx+b
Where m = slope and b = y-intercept.
So basically, you just plug in the missing values.
First, find the slope.
Equation for slope is : = y₂-y₁ / x₂-x₁
Plug in the missing values.
m = 4-9 / 2-12
m = -5 / -10
m = 1/2
Plug in 1/2 for m in y = mx + b
y = 1/2x + 9
It would be 9 because you just take the y-value of the first set of points.
~Hope I helped!~