Since it's in equilibrium, if you know the mass you can find the force the spring exerts. What happens if you put a larger mass on the end of the spring?

And how do you find the spring constant? I always recommend that students make a graph of some type of linear function and find the slope of that line. In this case, they could plot T2 vs. the mass.

The gradient of the graph of force (y-axis) and extension (x-axis) is numerically equal to the spring constant.