Not going to do (1), (2), (3) since they are implicit from (a) and (b) and require graphs (which aren't supported on brainly).
We have [tex]g(x,y)=\cos(x+2y)[/tex], that means, if plug in [tex]x=2,y=-1[/tex] we get [tex]g(2,-1)=\cos(2+2(-1))=\cos(0)=\boxed{1}[/tex].
Doman of cosine is [tex]x\in[0,1][/tex].
Hope this helps.