The decomposition of HI(g) is a second order reaction. 2 HI(g) → I2(g) + H2(g)

Above is some data from an experiment in which HI was decomposed. 

Using the reaction data above:
(a) If you wanted to create a linear graph for this experiment with time on the x-axis, what variable would you put on the y-axis?
(b) Use the data above to calculate k; include units.
(c) How many seconds will it take for the concentration of HI to reach 0.050 M?
(d) What will be the concentration of HI after 50.0 seconds?

Please I need this ASAP I’m stuck