Your answer is quite simple, really. All you have to do is divide $3.36 by 7 (The ounces of grapes he bought), and your answer should come out to be $.48, to check if this is correct, you then should add .48, 7 times, or multiply it by 7. Which, is what I did. And that should bring you to 3.36!
When you have

you can solve the equation by multiplying both sides by 1/4. Then simplify.

So, multiply the terms with equal exponents by multiplying their bases.
So, (2^7 × 4^7)^3 becomes [(2×4)^7]^3
Then, just multiply the numbers.
(8^7)^3
Then, simplify the expression by multiplying the exponents. So, the answer should be 8^21 or eight to the 21st power.
62,497,264
or 6.2497264 x 10^7
sixty two million four hundred ninety seven thousand two hundred sixty four