The sum of x and 3/5 is 5/7. what is the sum of x

Subtract 3/5 from both sides

x = 5/7 - 3/5

Simplify 5/7 - 3/5 to 4/35

x = 4/35

To solve this we need to move the decimal to get greater then 1 and less then 10.
So to do this move the decimal over (the right)
Now we have 0.07 = 7x10^-2
We had to move the decimal 2 places over to get 7 and since we moved it to the right, its going to be a negative.
Your answer is 
0.07 = 7x10^-2