We say that their extension of the each spring is same, so the equivalent spring constant k is the sum of all k of spring. But how can this make sense if one spring in a system of two springs, has a very large k1, and one has a very small k2? Wouldn't the system, like a load,when force is applied, "turns over" and "slanted" ？
Yes; a resultant torque causes rotation. If you don’t see this addressed in a problem, it’s because the block is being assumed to translate only, not rotate (e.g., perhaps it’s riding on frictionless tracks).
Other likely assumptions for this problem are that the wall and block are rigid, the springs are ideal, the system doesn’t sag from gravity, and that no out-of-plane motion occurs, for instance. Sometimes the problem will state these assumptions, and sometimes you have to deduce and apply them through experience.