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