You divide by 5/13. Remember when dividing fractions you multiply by the reciprocal. 5/13 cancels on the left side, leaving t.
So here you have -9/(5/13), but we can put -9 as -9/1, then instead of dividing by 5/13, multiply by 13/5: 117/5. This evens out, when divided, to t=23.4
