Solve {l}{4(I_{1}-I_{L})+1(I_{3}-I_{2})+5I_{3}=18}{2f_{2}+1(I_{L}-I_{3})+u(I_{2}-f_{1})=90y}

$$\left. \begin{array} { l } { 4 ( I _ { 1 } - I _ { L } ) + 1 ( I _ { 3 } - I _ { 2 } ) + 5 I _ { 3 } = 18 } \\ { 2 f _ { 2 } + 1 ( I _ { L } - I _ { 3 } ) + u ( I _ { 2 } - f _ { 1 } ) = 90 y } \end{array} \right.$$

Solve for I_1, I_L, I_3, I_2, f_2, u, f_1, y

$$y\in \mathrm{R}$$

$$I_{1}=f_{1}u-I_{2}u+\frac{I_{2}}{4}-\frac{I_{3}}{2}+90y-2f_{2}+\frac{9}{2}$$

$$I_{L}=90y+f_{1}u-I_{2}u+I_{3}-2f_{2}$$

$$I_{3}\in \mathrm{R}$$

$$I_{2}\in \mathrm{R}$$

$$f_{2}\in \mathrm{R}$$

$$u\in \mathrm{R}$$

$$f_{1}\in \mathrm{R}$$