Show that Euler's theorem implies that, for a constant returns$-$to$-$scale production function $q=f(k,l),$

$q=f_k*k+f_l*l.$

Use this result to show that, for such a production function, if $M{P}_l>A{P}_l$ then $M{P}_k$ must be negative. What does this imply about where production must take place? Can a firm ever produce at a point where $A{P}_l$ is increasing?

Please explain it from very basic