I believe I showed you how to answer this question before. But in case you did not read my answer, I will guide you through the steps.
You are going to set the inequality
area of shaded region ≥ 12
and you know that
area of shaded region = area of entire figure - area of unshaded region
I will assume that the shaded region is the boundary between the rectangle and triangle.
area of shaded region = area of rectangle - area of triangle
= (length *width) - area of triangle
= 3r - area of triangle
3r - area of triangle ≥ 12
3r ≥ 12 + area of triangle
r ≥ 4 + (area of triangle / 3)
You did not give dimensions of triangle, unless the answer is supposed to be in this general form.