y= mx+ b is the slope intercept equation form
You simply have to find the values for m (the line's slope) and the b (the line's y-axis intercept).
I have to assume that (2,1) is an ordered pair in the cartesian coordinate system that exists on the line which you are trying to find the slope intercept equation of.
If it is, then at that point on the line, the x value is 2 and the value for y is 1.
So you were given three of the four values (y, m, x, and b) needed to determine your equation.
If you substitute three known values into the slope intercept form of the equation for the line, then you will be able to determine the fourth unknown value.
In your case, after substitution, the generic equation y = (m)(x) + b becomes
1 = (-4)(2) + b
and after one step of simplification the equation becomes
1 = -8 + b
Next add 8 to both sides of the equation to isolate the b variable.
9 = b
Now you have the values for both the slope m and the intercept b which are always needed to write a line's equation in the slope intercept form (y= mx+ b).
You would then write the answer equation as:
y = -4x + 9