a) $\dfrac{x}{6}=\dfrac{-1}{2}$
$⇔x.2 = 6.(-1)$
$⇔x.2 =-6$
$⇔x=-3$
b) $\dfrac{x-7}{16}=\dfrac{9}{24}=\dfrac{3}{8}$
$⇒(x-7).8 = 16.3$
$⇒8x-56 = 48$
$⇒x=13$
c) $\dfrac{x}{7} =\dfrac{x+16}{35} $
$⇔35x = 7(x+16)$
$⇔35x-7x = 112$
$⇔28x = 112$
$⇔x=4$
d) $\dfrac{x}{4} = \dfrac{x+1}{8}$
$⇔8x = 4x+4$
$⇔4x=4$
$⇔x=1$