What does the slope of a velocity (or speed) vs. time graph mean? Physics homework help

Compare the slope of the velocity-time graph to the average of all your acceleration values. Are they close? What does the slope of a velocity (or speed) vs. time graph mean? Explain the answer using your data. How does the value of g that you calculated compare to the accepted value of 9.80 m/s²? What is your percent error? Remember that the value of g can be calculated by using g = a/sinθ and percent error can be calculated using the following equation 

the equation is % error= ((measured value-accepted value)/accepted value) x 100

I tried solving this but only got this:

% error= (0.1846 – 9.8)/9.8) x 100

I uploaded the data table from my experiment and i hope somone would lend me some help because ‘;m stuck!