For the two expressions to be equivalent, then we would have;
[tex]bx+2.4=6(2x+0.4)+3x[/tex]We solve the right hand side and we'll now have;
[tex]\begin{gathered} bx+2.4=12x+2.4+3x \\ bx+2.4=15x+2.4 \\ \text{Collect all like terms and you'll have} \\ 2.4-2.4=15x-bx \\ 0=15x-bx \\ Add\text{ bx to both sides of the equation} \\ 0+bx=15x-bx+bx \\ bx=15x \\ \text{Divide both sides by x} \\ \frac{bx}{x}=\frac{15x}{x} \\ b=15 \end{gathered}[/tex]The first option is the correct one.
b = 15
