The original post said "two consecutive mile posts". So, drawing the line from the balloon perpendicular to the road, we have two right triangles, one with angle 90- 16= 74 degrees, "near leg" of length h miles and "opposite leg" of length x miles. tan(74)= x/h. The second right triangle has angle 90- 12= 78 degrees, "near leg" of length h miles and "opposite leg" of length 1- x miles. tan(78)= (1- x)/h. You have two equations in x and h.