Sensitivity to initial conditions is well illustrated by a little target practice with your numerical solver. Enter $x' = x^2 - t$ into your numerical solver, and then experiment with initial conditions at $t_0 = 0$ until the solution with $x(t_0,\ x_0)$ hits the target $(5,\ 0)$.
I don't fully understand what the goal is here or what is being asked. I'm uncertain of what the textbook means by "numerical solver." In other contexts, I understanding it to mean a numerical method algorithm, but it seems to be referring to software, perhaps software that graphs the slope field of the given ODE? Which of the following qualify as numerical solvers, and for each that does, outline the steps one would go through to complete the exercise and understand the concept being illustrated?