if the slopes are the same and the y-intercepts are different, the lines are parallel. If the slopes are different, the lines are not parallel. Unlike parallel lines, perpendicular lines do intersect.
So in this systems of equations, we know the prices of the items, so the coefficients will represent how many Mary is getting of that item. So you can represent amount of bread as the variable b and the amount of milk as the variable m. On the right will be the total amount spent, since multiplying the price by how much you buy should get you the price
So since at store A, she spends 44.20 and bread costs 2.20 and milk costs 3.60 you get
Since at store B, she spends 44.70 and bread costs 2.50 and milk costs 3.40 you get the equation:
Because if it the cost of each your mulitplying and because you wanting to just define the variable p for pen and n for notebook all a variable is a letter than let you know what something is for using in math later