Define Proportion

If four numbers $a, b, c,$ and $d$ are in proportion, they satisfy the relation $\frac{a}{b} = \frac{c}{d}$. Here, $a=7, b=12, c=21, d=k$.

$\frac{7}{12} = \frac{21}{k}$

Cross-Multiplication

Cross-multiply to solve for $k$: $7 \times k = 12 \times 21$.

$k = \frac{12 \times 21}{7}$

Calculate Value

Divide $21$ by $7$ to get $3$, then multiply by $12$: $k = 12 \times 3 = 36$.

$k = 36$