Answer:
For an equation like:
y = a*x + b
a is the slope, and this is the rate of change of the function.
For the case of the problem, we know that the equation represents the amount of oxygen in the spaceship as a function of time. Where as the time passes, the quantity of oxygen decreases.
Then the slope is the rate of change that tells how the quantity of oxygen changes as time passes. Then the slope is the amount of oxygen that is consumed per unit of time.