Use the slope formula to solve this.
m = y₂ - y₁ / x₂ - x₁
So since the points are (4, 1) and (7, -2) here's how you'd work it out:
Substitute the variables for the corresponding number.
m = -2 - 1 / 7 - 4
Solve the top and bottom.
m = -3 / 3
Now divide.
m = -1
So your answer is B.