Answer
$b=-\frac{2}{3}$
Work Step by Step
Use cross multiplication since the variable is in the numerator and denominator.
$\frac{3b}{b-4}=\frac{3}{7}$ cross multiply
$21b=3(b-4)$ distributive property
$21b=3b-12$ subtract 3b from both sides
$21b-3b=3b-12-3b$ combine like terms
$18b=-12$ divide both sides by 18
$18b\div18=-12\div18$
$b=-\frac{2}{3}$
