To estimate the height of a tree, a forester uses the concept of similar triangles. A particular tree is found to have a shadow measuring 84 feet in length. The forester has her own shadow measured at the same time. Her shadow measures $7 \frac{1}{2}$ feet. If she is $5 \frac{1}{2}$ feet tall, how tall is the tree? (FIGURE CAN'T COPY).