Answer:
Length of BH is 2 units.
Explanation:
We have, ΔABC with ∠ABC = 90° and an altitude BH.
Also, it is given that the length of sides AH is 4 units and HC is 1 units.
Since, the 'Altitude Rule' states that 'the length of the altitude is the geometric mean of the line segments it bisects'.
So, we get that,

i.e.

i.e.

i.e.

Since, the length of the side cannot be negative.
So, length of BH is 2 units.