It just so happened that I found the answer this way, but I'm not sure if this is how you should solve this problem.
So I tried to find the slope intercept form, which is y = mx + b.
"m" is the slope, "b" is the y-intercept (the y value when x = 0)
Since you know the slope is 3, you can plug it in.
y = mx + b -----> y = 3x + b
To find "b", you plug in the point you know, which is (10,9) into the equation
y = 3x + b
9 = 3(10) + b
9 = 30 + b
-21 = b
Your equation is y = 3x - 21.
Next I used the slope formula and plugged in what I know:
[tex]m=\frac{y_{2}-y_{1}}{x_{2}-x_{1}}[/tex]
[tex]3=\frac{b-9}{a-10}[/tex]
Since you are missing a point, I used a point that we found, which was the y-intercept: (0, -21), and plugged it into the equation.
[tex]3=\frac{-21-9}{0-10}[/tex]
[tex]3=\frac{-30}{-10}[/tex]
3 = 3
Your point (a,b) is (0, -21)
a = 0
b = -21
