looking at the denominators, since 7 is a prime number and 18 has factors of 2*3*3, the LCD will simply be the product of 7 and 18.[tex]\bf \cfrac{17}{18}-\cfrac{3}{7}\implies \stackrel{\textit{using the LCD of 126}}{\cfrac{(7)17~~-~~(18)3}{126}}\implies \cfrac{119-54}{126}\implies \cfrac{65}{126}[/tex]