Remember the constant of integration for c? In order to get a particular solution, it involves an initial condition to find specific value of C. (i.g. f(a)=b)

Once we found the general solution, we can use the condition to yield out the value.

For example

$\int x^2 dx=\frac 1 3 x^3+C=f(x)\newline f(x)|_{x=3}=3 \newline \because \frac 1 3 (3)^3+C=3 \therefore9+C=3 \rightarrow C=-6$