Eight times the difference of a number and seventeen is thirty

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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 

There are two bc a studen that equals to 2