Well, first of all, rearrange things to express them in the same form

5x - 3y = 10

kx - 6y = 42

Do you know how to solve a system of equations in general? Try using each

equation to eliminate one of the variables, by expressing it in terms of all the others. For example, if you solved the first equation for 'x', you could then substitute that expression for 'x' into the second equation, and everything would now be in terms of only one variable: y. You would be able to solve the second equation for y, but presumably there would be some value of k for which you wouldn't be able to obtain a solution to the second equation.

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