The easiest way to know which tree is growing fastest is to find the slope of each tree growth.
Slope of tree A is already given, and is 4
For the slope of tree B, use two of the pairs given on the table.
Similar procedure is done for the slope of tree C, use two points on the graph to find the slope.
The greatest slope is 4.5 and corresponds to tree B, it means tree B is the one that is growing fastest.
Tree A was 5 inches tall when first planted, tree B was 3 inches and tree C was 10 inches. It means the tallest tree when it was first planted as tree C.
After 6 months, tree A will be (4*6)+5=29 inches.
After 6 months tree B will be (4.5*6)+3=30 inches.
After 6 months tree C will be 31 inches.
It means that after 6 months, tree C will be the tallest of them.