Pleas explain step by step

$[20$ pts$]$The additive noise over a binary transmission line has the following pdf:

Transmitter sends $5$ V for bit $"1"$ and $0$ V for bit $"0".$ If Pr$($ transmitting $"1")=0\mathrm{.}4$ and

Pr$($ transmitting $"0")=0\mathrm{.}6,$ and the decision threshold at the receiver is $2\mathrm{.}5$ V $,$ calculate the

probability of bit error.