We are asked to express 0.004 in scientific notation
The number 0.004 has the decimal point at the start, so we move this decimal point to the right until there is only one non-zero digit is left (4 in this case) and then count the number of times we moved.
In this case, we moved 3 times so the exponent (power) is -3
The sign of exponent is negative when we move to the right (like in this case)
The sign of exponent is positive when we move to the left.