syms I1 I2 I3 rl
v1=10;
v2=5;
v3=4*I1;
r1=6;
r2=20;
r3=15;
r4=6;
r5=10;
eqn = [0==v1-(r1*I1)-(r2*I2),0==(r2*I2)-(r3*(I1-I2))+v2-(r4*I3)-(v3),0==(v3)+(r4*I3)-((r5+rl)*(I1-I2-I3))];
[I1, I2, I3] = solve(eqn, [I1, I2, I3]);
fun = matlabFunction([I1, I2, I3]);
You can now call fun() with a scalar or column vector argument of rl values. The corresponding I1, I2, I3 values will be returned across the columns.