It is given in the question that
Suppose the supply function for product x is given by
[tex]qxs = - 30 + 2px - 4pz.[/tex]
And we have to find how much of product x is produced when px = $600 and pz = $60.
And for that, we have to substitute 600 for px and 60 for pz, and on doing so, we will get
[tex]qxs = -30+2(600)-4(60) \\ qxs = -30 +1200 -240 = 930[/tex]
And that's the required answer .