I have forgotton how to change a decimal into a fraction can you please give me the formula.

for example for:

-2.75

You just have to move the decimal two places to the right and put it over 100.

If you multiply a number by some factor and divide it by the same factor it stays the same. In this case multiply and divide by 100. Then you got a fraction. Then simplify that fraction by dividing out common factors in the numerator and denominator.