Please help due today {Mark brainlist} 100 points

How do you determine whether a relationship represented as as graph is linear or nonlinear

Solve for x and y: 217x + 131y = 913; 131x + 217y = 827

Which is the correct first step in finding the area of the base of a cylinder with a volume of 26x cubic meters and a height of 6.5 meters? v=bh 6.5=b(26x) v=bh v=26pi+(6.5) v=bh v=26pi(6.5) v=bh 26pi=b(6.5)