The expression of a line is given by the following expression:
[tex]y\text{ = m}\cdot x\text{ + b}[/tex]In this problem we were given the value of m and a point that belongs to the line. If we use both on the expression above we will find the value of b.
[tex]\begin{gathered} 8\text{ = 5}\cdot3\text{ + b} \\ 8\text{ = 15 + b} \\ b\text{ = 8 - 15} \\ b\text{ = -7} \end{gathered}[/tex]The value of "b" is -7 and the expression is:
[tex]y\text{ = 5}\cdot x\text{ -7}[/tex]