You will need to make b the subject of the formula:
This you do by subtracting 0.25 from both sides of the equation:
b + 0.25 = 1.15 becomes:
[tex]\begin{gathered} b+0.25-0.25=1.15-0.25\text{ which turns to become:} \\ b=1.15-0.25 \\ b=0.9 \end{gathered}[/tex]