ma'am my query is how to find instantaneous velocity at an instant.
This is similar but It is simpler:
Step 1 -- From the distance-time graph, if you know what time you are particularly in, go to your time axis (should be the horizontal axis) and from this particular time (point) on the time axis, go vertically upwards until it intersects the graph. Step 2 -- At this point of intersection, go horizontally to the left and read the value of the point at which this horizontal line intersects the vertical axis. This is the distance travelled by the body/particle at the particular instant chosen. Step 3 -- To determine the instantaneous velocity, simply divide the distance (determined in Step 2) by the time (from Step 1).