Can someone me understand how to create 3 proofs

X+ y = 5 ; x - y =3 solve using linear combination

Find the domain of the following expression: 1/(√x)-2

The revenue each season from tickets at the theme park is represented by t(c)=5x. the cost to pay the employees each season is represented by r(x)=(1.5)^x. examine the graph of the combined function for total profit and estimate the profit after four seasons