# Solving differential equations using summation of singularity functions

I'm stuck at Problem 2.56 of Alan Oppenheim's Signals and Systems book. What is the method called? Where or which book can I learn more about it? It assumes $y(t)=\displaystyle \sum_l \alpha_l u_l(t)$. I didn't learn it while studying basic differential equations.
$u_o(t)=\delta(t)$
$u_1(t)=\frac{d \delta(t)}{dt}$