The slope of the indifference curve is known to be A. The marginal rate of substitution is usually done when the value of y is given up in order to improve the value of x. This type of graph may be used by companies who would like to see the correlation of their goods to the satisfaction of customers. If you cannot understand this, you can liken it to eating a burger.
The first burger that you will consume will be very nice to eat. It will satisfy you greatly. When you eat your fifth burger, you will not be too satisfied with what you are eating anymore. This is what the marginal rate of substitution shows.