eight times the difference of a number and seventeen is thirty

OK.  Great.  Thanks for sharing.  Do you have a question to ask ?

My hunch is that you want to know what the "number" is.
Here's how to set it up and solve it:

"a number"  . . . . . . . . . . . . . . .    Q

"The difference of 
a number and 17"  . . . . . . . . . .   Q - 17

"eight times the difference"  . . .  8 (Q - 17)

"is thirty"  . . . . . . . . . . . . . . . . . . 8 (Q - 17)  =  30  


Eliminate parentheses on the left side:

                                                    8Q - 136  =  30

Add  136  to each side:               8Q           =  166

Divide each side by  8 :                 Q           =   20.75