Triple a number b is greater than or equal to 21

What is the slope of the line shown below?

The vet charges $45 for an office visit and $10 for each vaccination your dog needs. write an eaquation

Lindsay used two points, (x,y; ) and (+2.82), to find the equation of the line, y = mx + b, that passes through the points. y2-y, first, she used the definition of slope and determined that the value of mis x, - . given this information, which expression must represent the value of b?