Answer: D
Step-by-step explanation:
The dotted line is the altitude, since this is an isosceles triangle, it spits the bottom line in half and forms a right angle so you can use the pythagorean theorem.
[tex]a^{2} +b^{2} = c^{2}[/tex]
[tex]a^{2} + (4)^{2} =(\sqrt{20})^{2}[/tex]
simplify
[tex]a^{2} +16=20[/tex]
subtract 16 from both sides
[tex]a^{2} =4[/tex]
square root both sides
[tex]a=2[/tex]
