136. Joint system. For which values of $k$ is the system given below consistent?

$$
\left\{\begin{aligned}
x+y & =1 \\
k x+y & =2 \\
x+k u & =3
\end{aligned}\right.
$$
136. Adding the second and third equations of the given system, we get

$$
x+y+k(x+y)=5
$$

Considering the first equation, we find from here the desired value $k=4$. For this $k$ we have $x=\frac{1}{3}, y=\frac{2}{3}$.