Open Question: Problem understanding domain and co domain in functions?

Hi friends,

I have some problems in maths for which I am not understanding the domain and the co domain.

a. Let A, B, C is a subset of R square. where A = { (x,y) , y=2x+1}
B = {(x,y) / y=3x}
C = {(x,y) / x-y = 7}

here in the above question what does it mean by saying “A, B, C is a subset of R square”. I do not understand the significance of R square here.

Please let me know.

Regards,
Pradeep

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