Write the signal f(t) shown in <image 1> using the step functions.
['4t^2 [u(t) - u(t - 5)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]', '4t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]', '5t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 5)] + 15(t - 7)[u(t - 5) - u(t - 7)]', '5t^2 [u(t) - u(t - 2)] + 20[u(t - 2) - u(t - 3)] + 15(t - 7)[u(t - 5) - u(t - 7)]']