Answer:
The balloons new height is 
Step-by-step explanation:
Let
x ----> the number of minutes
y ----> the height of a hot air balloon in feet
we know that
The linear equation in slope intercept form is equal to

where
m is the slope or unit rate
b is the y-intercept or initial value
In this problem we have
The slope is equal to
---> is negative because is a decreasing function
The y-intercept is equal to

substitute

For x=9 minutes
substitute in the linear equation

