How do I get the spring constant, k, from the equation for a trend line?

First of all, Hooke's law only applies to a spring that is no stretched out. The actual graph in real life (not ideal) is a linear line first, then it becomes a curve to a limit point. The equation you got there is not very convincing. Because of the closer the y-intercept to 0, the more accurate your data is. Because the starting point is (0,0). you might want to check the r (coefficient of correlation) to see how close r to 1. Assuming your data is relatively correct, we can ignore the y-intercept and use slope as the spring constant, AKA the stiffness coefficient, which is 0.0253.