Mechanics - acceleration

So there is one question where a straight-line graph is plotted and I am asked to find the acceleration using it. ( on the y-axis is time^2 in seconds^2 and on the x-axis is the distance in m)

The gradient of the line is 0.21 (this question was on a test and I do not have a picture of the question, but I remember this)

the equation is:

t = 2s/g

using the gradient and points from the graph, calculate what g is.
(I do not remember any points from the results table, lets called any given points a and b)

How would you find out what g is using the gradient any points from the graph/results table?

$s=ut+\frac{at^2}{2}$

A straight line would imply u=0. $\therefore y=t^2=..$

So it would be s = 0.5a * t^2
and if I arrange it I would get a = 2s/t^2?
How is the gradient involved because it asks me to use the gradient as well, which is t^2/s (seconds^2/meters)

Put it in y=mx form to get the gradient.