Answer:
Let x be distance of bottom of ladder from wall
Let H be distance of floor to top of ladder
x^2 + H^2 = L^2 Pythagorean theorem
2 x dx/dt + 2 H dH/dt = 0 L is length of ladder and is fixed
dH/dt = - x/H dx/dt
x = 3 given H = 4 to complete theorem
dH/dt = -3/4 * .5 m/s = -.375 ft/sec rate of height of ladder changing