We can use proportional reasoning to solve some questions directly, such as which size of laundry detergent is the cheapest per load, or what the dimensions of the model car should be.
Recall that 
, so we can write everything in terms of 
:
Let 
, so that
With some rewriting we get
Clearly we cannot have 
, or 
.
The numerator determines when the expression on the left reduces to 0:
Completing the square gives
so that
The second equation gives no real-valued solutions because squaring any real number gives a positive real number. (I'm assuming we don't care about complex solutions.) So we're left with only
which again gives two cases,
Then when , we can find 
by taking the reciprocal, so we get
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.
