i think that i should use like law of sines of cosines but I'm not sure, could I get some help, really don't know where to start...

Nah

If we know an included angle between two known sides, the area =

(1/2) ( product of the sides)* sine (of included angle)

So

(1/2) (6 * 6) sin 45°

Can you take it from here ???

yes, thank you!