The ladder forms the hypotenuse of the right angle, with the base being the distance from the foot of the ladder to the base of the wall and the height being the height of the top of the ladder from the foot of the wall.

By the Pythagoras Theorem.
a^2 + b^2 = c^2
a^2 = c^2 - b^2
a^2 = 5^2 - 1.8^2
a^2 = 21.76 m
a = sqrt 21.76
a = 4.664761516 m
a ~ 4.66 m

Answer seems really off. I suspect a typo of "3" instead of the correct "4".