The only thing that can be varied is scope. Do more, take longer. Increasing Quality requirements makes Time increase. Do better, take longer. Increasing available Resources can sometimes make Time decrease.
Reverse relationships hold.
If Management and the Customer exercise control over Scope, Quality, and Resources, then Time is completely determined by the dynamics of the development process. It is a matter of measuring progress and predicting the final result. You hold your nose and do the math. Conclusions Learn how to measure and predict your progress. In a context of short iterations with defined goals, this is actually fairly easy.