a) $3×x+7=19$
$3×x$ $=19-7$
`3×x` `=12`
`x` `=12÷3`
`x` `=4`
b) $\dfrac{2}{3}$ $÷$ $(2\times x-1)+$ `1/4=9/4`
$\dfrac{2}{3}$ $÷$ $(2\times x-1)$ `=9/4-1/4`
$\dfrac{2}{3}$ $÷$ $(2\times x-1)$ `=2`
$2\times x-1$ `=2/3÷2`
$2\times x-1$ `=1/3`
`2\times x` `=1/3+1`
`2\times x` `=4/3`
`x` `=4/3÷2`
`x` `=2/3`