You'll have to either transform 12.8 to a fraction or 3/4 to a decimal number. I'll transform 3/4 to a decimal:
3/4 = 0,75
After that we do the multiplication normally:
12.8 x 0,75 = 9,6
If you want, you can tranform 9,6 into a fraction:
=

Hope it helped,
BioTeacher101