The unknown side is the hypothenuse of the triangle.
We use the Pythagoras theorem which states that the square of the hypothenuse equals the sum of the opposite square and adjacent square.
Mathematically, the Hypothenuse is:
[tex]H=\sqrt[]{O^2+A^2}[/tex]
where H = hypothenuse,
O = opposite
A = adjacent
Thus, Hypothenuse in this instance is:
[tex]H=\sqrt[]{2^2+4^2}=\sqrt[]{20}=\text{ 2}\sqrt[]{5}[/tex]
