First we have to divide both sides of the equation by 1/2a:
[tex]\frac{c}{\frac{1}{2}a}=\frac{\frac{1}{2}a}{\frac{1}{2}a}b^{2}[/tex]And we get
[tex]\frac{2c}{a}=b^{2}[/tex]finally we have to take square root on both sides:
[tex]\sqrt[]{\frac{2c}{a}}=\sqrt[]{b^2}[/tex]And we get that b is:
[tex]b=\sqrt[]{\frac{2c}{a}}[/tex]