Any time you are ading or subtracting fractions you want to find like denominators. So first list the multiples of each denominator until you find the smallest number they have in common.

At my school the younger children call it skip counting. 4,8,12

Do the same with the 3=3,6,9,12. So 3and 4 have least common denominator or 12. then you take each fraction and multiply both numbers that gave you the least common denominator.

-5/4 x 3/3= -15/12 do thesame with the second fraction 2/3 x4/4 = 8/12

now you have -15/12 - 8/12 this is the same as -15/12 + -8/12 = -23/12 or -1 and11/12 as a mixed number.