Question is slightly wrong .We have to find BD
we know
[tex]\boxed{\sf BD^2=AD\times DC}[/tex]
[tex]\\ \qquad\quad\sf{:}\twoheadrightarrow BD^2=4\times 2[/tex]
[tex]\\ \qquad\quad\sf{:}\twoheadrightarrow BD^2=8[/tex]
[tex]\\ \qquad\quad\sf{:}\twoheadrightarrow BD=\sqrt{8}[/tex]
[tex]\\ \qquad\quad\sf{:}\twoheadrightarrow BD=2√2[/tex]
[tex]\\ \qquad\quad\sf{:}\twoheadrightarrow BD=2(1.4)[/tex]
[tex]\\ \qquad\quad\sf{:}\twoheadrightarrow BD=2.8[/tex]