To find out what a fraction as a decimal would be, you simply need to divide the numerator by the denominator.

Now to find the percent.
To find a percentage based off of a decimal, we can either multiply the decimal by 100, or (my personal shortcut) just shift the decimal point over two places to the right.
-0.9 = -90%
which is all we need to know.
written as a decimal and a percent is -0.9 and -90% respectively.
Hope that helped! =)