Answer
$$y = -5{e^{ - 3t}} + 10$$
Work Step by Step
$$\eqalign{
& y = C{e^{ - 3t}} + 10 \cr
& {\text{The initial condition is }}y\left( 0 \right) = 5,{\text{ then}} \cr
& y\left( 0 \right) = C{e^{ - 3\left( 0 \right)}} + 10 \cr
& 5 = C\left( {{e^0}} \right) + 10 \cr
& 5 = C + 10 \cr
& C = -5 \cr
& {\text{The solution is }} \cr
& \underbrace {y = C{e^{ - 3t}} + 10}_{C = -5} \cr
& y = -5{e^{ - 3t}} + 10 \cr} $$