Given E=mc2E = mc^2E=mc2, solve for ccc (assume c>0c > 0c>0).
c=Emc = \sqrt{\frac{E}{m}}c=mE
c=Em2c = \frac{E}{m^2}c=m2E
c=Emc = \frac{\sqrt{E}}{m}c=mE
c=mEc = \sqrt{\frac{m}{E}}c=Em