Who's cost is this? Is that the price a store pays for something? If so, then you just have to divide the regular price by 1.7.

To see why, remember that to mark it up 70%, the store multiplied the cost by 1.7. Algebraically,

P = 1.7*C

So, to go back to the cost from the price, just do the opposite: divide by 1.7. Algebraically, you solve for C:

C = P/1.7