We know x is the original cost per anorak.

Let a be the amount of anoraks purchased.

Let T = total cost

From this we have

Now, originally, you would have:

But now, cost has risen by six, so amount must decrease by five. Remember that the new cost * new amount still = 600.

But what is a? Well, we can put a in terms of x.

So we have the equation:

Hopefully you can take it from there