We assume the total time to be $‘t’$ units and $f1$ executes for $‘x’$ units.
Since, $f1(t)$ and $f2(t)$ are executed sequentially.
So, $f2$ is executed for $‘t – x’$ units.
We apply convolution on the sum of two independent random variables to get probability density function of the overall time taken to execute the program.
$f1(x) $*$ f2(t – x)$