Equivalent for parallel combination of $5 \mathrm{\Omega } \& 15 \mathrm{\Omega }$ is 

$R_{\mathrm{eq}1}=\frac{15\times 5}{15+5}=\frac{15}{4} \mathrm{\Omega }$

Similarly for three parallel resistor's equivalent 

$\frac{1}{R_{\mathrm{eq} 2}}=\frac{1}{24}+\frac{{\displaystyle 1}}{{\displaystyle 12}}+\frac{{\displaystyle 1}}{{\displaystyle 8}} \Rightarrow R_{\mathrm{eq} 2}=4 \mathrm{\Omega }$

Now current through $45 \mathrm{V}$ branch,

$I=\frac{45}{R_{\mathrm{eq} 1}}=\frac{45}{15/4}=12 \mathrm{A}$

Voltage across $a \& b$ is

$V=R_{\mathrm{eq}}\times I=\left(4+10+\frac{15}{4}\right)\times 12=213 \mathrm{V}$