Just substitute M=4.2 in the given equation. Make the whole log term the subject.
[tex]\log(\frac{E}{10^{11.8}})=\frac{3M}{2}[/tex]
Then, remove the log,:
[tex]\frac{E}{10^{11.8}}=10^{\frac{3M}{2}}
E=10^{11.8}\times10^{\frac{3\times4.2}{2}}=10^{11.8+6.3}=10^{18.1}=1,258,925,411,794,167,210[/tex]