Since the 2 fract. have diff. denoms., we must find the LCD.
Note that factors of 15 are 3 and 5, and that
factors of 25 are 5 and 5. The LCD thus includes 5, 3 and 5, and is 75. Notice that both 15 and 25 divide evenly into 75.
Thus we have
3*3
15/75 - 6/75, or -9/75. This must be reduced; the answer is - --------
3*25
and this reduces further to 3/25 (answer).