let T = the height of the tree
because the proportion of the man's height to his shadow is equal to the tree's height to its shadow we can write the equation
T/14 = 6/3.5
now multiply both sides buy 14*3.5 and we get 3.5*T = 6*14 or T = 84/3.5 = 24
hence the height of the tree is 24 feet