Integral equation $u(t)=f(t)+a\int_0^t u(s)ds\quad t\geq 0$

Integral equation $u(t)=f(t)+a\int_0^t u(s)ds\quad t\geq 0$

Let $a\in R$ e $f\colon [0,1]\to R$ a continous function. Solve the
integral equation $u(t)=f(t)+a\int_0^t u(s)ds\quad t\geq 0$ and find an
explicit formula for the solution. Thank you