Looks okay to me. What does she think it ought to be? Maybe it's the bit that leads up to this what's wrong.
What was the question? The only thing "wrong" with that is that it doesn' maqke any sense at all! Why should "b" have anything at all to do with x and y? What are you trying to do?
Frankly, it looks to me like you have a very good teacher who is concerned about your tendency to do algebraic operations without thinking about what you are doing.
The problem may be have this given values, slope = m = -2; then the line passes through points (-2,-7). find the equation of the line.
y = mx + b, where m = -2 and points (-2,-7), substituting
(-7) = (-2)(-2) + b
(-7) = (4) + b
-11 = b
y = -2x -11. This is the slope-intercept form and seems alright.
What if you have interchanged the point (-7,-2) to (-2,-7)?
Or you have the wrong form of the equation of the line . . . .