I could be wrong but ... I will try to answer
you first find the common factor
4:3 and 9:5
then
4 times 3 is 12
5 times 3 is 15
the ratio is now:
12:9:15
add the numbers up and that would be 36
divide 754 by 36
which is about 20.94444....
multiply 20.94 by 12, 9, and 15
12 would be 251.28
9 would be 188.46
15 would be 314.1
add them together and they would be about 753.84