What is the equation for time when using a position-time graph?

and

What is the equation for displacement when using a velocity-time graph?

This really depends on the graph but to approach these types of problems you can go about it this way (check explanation) ...

Explanation:

(1) What is position?

Where is the object's location at?

If slope is zero, velocity is zero (straight line)

Slope positive, velocity is constant / positive (linear line positive)

Object is accelerating ... curve upwards.

Steeper the slope, higher the speed

(2) What is velocity?

What are velocity vs. time graphs? - Khan Academy