How do I determine if each equation is non linear? Which ones are linear, and which ones aren't?

y = x/2 - 19, 3y = x^2, 13y = 1/3x + 5, y = x + 25/5, y^3 = x

Restrictions on f(x) = 1/x+5 and g(x) = x-2

If the wwight of a package is multiplied by 7/10 the result is 65.8 find the weight of the package

Barney & noblet customers can choose to purchase a membership for $25 per year members receive 10% off all store purchases 1. how much would a member pay per year if he bought $50 worth of items each year? 2. write a slope-intercept equation that expresses how much a member pays per year using x to represent his purchases per year 3. how much would a member have to spend per year to pay off his membership fee?