Simple circuit analysis with node potential method
I am going to solve the circuit by using nodal analysis. The sum of the currents out of node A must be 0. 
\begin{align*}
\frac{V_a - 90}{5} + \frac{V_a}{5} + \frac{V_a - 100}{25}  &amp;= 0 \\
5(V_a - 90) + 5V_a + V_a - 100 &amp;= 0 \\
11V_a &amp;= 550 \\
V_a &amp;= 50 \\
I_3 &amp;= \frac{50-100}{10+15} = \frac{-50}{25} \\
I_3 &amp;= -2
\end{align*}