What is the length of side ab of the triangle?

Start with the Pythagorean theorem{A²+B²=C², A and B are the sides and C is the Hypotenuse/Longest Side.}
2²+4²=C²
4+16=C²
20=C²{now square root both sides.√}
√20=√C²
4.472= C

P.S.: hope I helped :)