423 times 28 in grid method

What is the point-slope form of the equation for the line with a slope of -2 that passes through (1,4)?

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?

Can someone show me step by step how to do this?