You have 2 unknowns so you need to find 2 simultaneous equations and solve them.

There are 2 ways to do this, the long way and the short way.

The long way to do this is to set up a lagrangian. You want to minimise the total cost, subject to the constraint that you have produced 121000 airframes.

Total cost: K + 10L

Constraint:

Lagrangian

If you solve this in the usual way, you will get 2 simultaneous equations:

Solve those to get your answers of L and K.

short cut

You only need the ratio L/K. You can get this from the second simultaneous equation on its own

Interestingly, for this production function, the ratio of K/L is constant at all levels of output. So you didn't need to know that there were 121000 airframes after all.