carbon have 6 e^-e−
=>⇒
1s^(2)2s^(2)2p^21s22s22p2 in normal state following klechkowski rule
1s^(2)2s^(2)2p_x^(1)2p_y^(1)2p_z^01s22s22p1x2p1y2p0z you can see the orbital 2p_z2pz have no electron
So the orbital 2s^22s2 give one electron this is called the hybridization state
you have now sp^3sp3 hybridization
BUT, to form double liaison you need piπ electron
piπ electron don't match with hybridization state, so the orbital 2p_z2pz get out
so finally you have sp^2sp2
---------------------−−−−−−−−−−−−−−−−−−−−−
You can done this without any complication using the VSEPR Theory
Here the carbon is : AX_3AX3 which means it's sp^2sp2