We can model the problem as a linear equation of the form:

Where:
m = Slope (Rate of change)
b = y-intercept (Initial value)
a)
Since it is descending at a rate of 25ft per minute, the slope is:

So, the equation is:

b) We know that the ballon was 425ft above the ground 21 minutes after it began its descent, so:

Therefore, the balloon was 950ft when it began its descent, so, we can conclude that the y-intercept is 950, now the equation is complete

c) We need to know for which value of x, y is equal to 0, so:

Solve for x:

The balloon will land after 38 minutes