6−8x=7x−10x+21

6−8x=−3x+21

6−8x+8x=−3x+21+8x

6=5x+21

6−21=5x+21−21

-15=5x

\(\displaystyle{\frac{{{15}}}{{{5}}}}={\frac{{{5}{x}}}{{{5}}}}\)

-3=x

6−8x=−3x+21

6−8x+8x=−3x+21+8x

6=5x+21

6−21=5x+21−21

-15=5x

\(\displaystyle{\frac{{{15}}}{{{5}}}}={\frac{{{5}{x}}}{{{5}}}}\)

-3=x