Why do you first combine like terms and then use distributive property to solve 8(5x+9x+6)=160?

You can actually do it either way, you could distribute first and then combine, you would get the same answer. I guess it just depends on what you find easier. Here is how to solve it both ways:

Combining First:

8(5x+9x+6)=160

8(14x+6)=160

112x+48=160

112x=112

x=1

NOT Combining First:

8(5x+9x+6)=160

40x+72x+48=160

112x+48=160

112x=112

x=1

so, you would get the same answer both ways, it is of personal preference what you do first.