Please help. 7/8 x c for c =8

If it says "for c = 8", it means you can just write "8" instead of "c".
So let's do that:

7/8 x 8.

There's a very simple rule for this.
Let's figure it out together.

Suppose you have half a pizza (1/2). I then give you another half a pizza. Then you have two halves of a pizza right? That's 1/2 x 2. That's a full pizza!

Now what if you have 1/3rds of a pizza? I give you another 1/3rds of a pizza, and your mother gives you another 1/3rds of a pizza. That's 1/3 x 3, and that's also a full pizza.

So we found:
1/2 x 2 = 1
1/3 x 3 = 1

Using similar logic we can say:
1/4 x 4 = 1
1/5 x 5 = 1

Etc.
Now. What if you have 2/3rds of a pizza. I give you 2/3rds of another pizza and your mom gives you 2/3rds of her pizza. That's 2/3 x 3. How many pizzas do you have in total? If you count correctly, you get 2 full pizzas.

So we get:
2/3 x 3 = 2.

In similar logic, we can say:
3/4 x 4 = 3
2/4 x 4 = 2
7/8 x 8 = 7.