Solve the proportion \[ \begin{array}{l}\frac{7}{21}=\frac{b}{18} \\ b=[?]\end{array} \]

To solve the proportion \(\frac{7}{21}=\frac{b}{18}\), we can cross-multiply. This means we'll multiply \(7\) by \(18\) and \(21\) by \(b\): \[ 7 \times 18 = 21 \times b \] Calculating \(7 \times 18\): \[ 7 \times 18 = 126 \] So the equation becomes: \[ 126 = 21b \] Now, we'll divide both sides by \(21\) to solve for \(b\): \[ b = \frac{126}{21} \] Calculating this gives: \[ b = 6 \] Thus, the value of \(b\) is \(6\).