Write a linear equation that describes the value y (in dollars) of the mri machine in terms of the time t (in years), 0 ≤ t ≤ 9.

For this case you could write a generic linear equation that determines the value in dollars after a number of years corresponding to the interval [0, 9]. Writing the equation we have
 y = a * t + b with t [0, 9]
 where,
 a and b with constants that belong to the real numbers
 t: it's time in years
 y: the value in dollars of the mri machine.
 
 y = a * t + b with t [0, 9]