Write 5 x 10^-3 as an ordinary number.

5 x 10⁻³ is a scientific notation.

5 is the coefficient
10 is the base
3 is the exponent. The sign of the exponent signifies which direction the decimal point moved.

negative sign means that the decimal point moved from left to right in writing the scientific notation.
positive sign means that the decimal point moved from right to left in writing the scientific notation.

5 x 10⁻³ is 0.005 in standard form.

had the exponent been positive.
5 x 10³ is 5000 in standard form

20y5 times 2 is 1010 times -2 is -20, plus y