Let B be the 3×3 matrix containing the cofactors, with each entry multiplied by the appropriate ± sign. That is, the ij-entry of B is (−1)i+j det(Mij ). Compute A·BT. You should get a diagonal matrix with the same number in every diagonal entry. In other words, a multiple of the identity matrix. What multiple is it (in terms of A)?

The answer is b

The slope of the graph is 1/3; it rises by 1 every 3. Rise/run = 1/3
The line is solid, and not dotted, so you know that the sign for the inequality is greater than or equal to, or less than and equal to.
Because the graph opens upwards, y>f(x)
The y-intercept is -1

When you combine all these factors, you get the function y >= (1/3)x - 1