start by getting like terms to one side.
x/9 - x/10 = -4
now make the fractions have a common denominator so that you can subtract
10x/90 - 9x/90 = -4
now simplify
x/90 = -4
now multiply each side by 90 to get rid of the fraction
x = -360
to check, just plug in the value of x into your original equation
it should come out to
-40 = -40