#### Answer

$-\frac{3x}{5}$

#### Work Step by Step

To write a fraction in simplest terms divide the numerator and denominator by the greatest common factor.
$42x=\underline2\times3\times\underline7$
$70=\underline2\times5\times\underline7$
$GCF=2\times7=14$
$-\frac{42x}{70}=-\frac{42x\div14}{70\div14}=-\frac{3x}{5}$