Find k so that the pair 2x + 3y = 6 and kx + 9y = 18 has infinitely many solutions.

For $a_1x + b_1y + c_1 = 0$ and $a_2x + b_2y + c_2 = 0$: if $\dfrac{a_1}{a_2} \neq \dfrac{b_1}{b_2}$ the lines cross; if $\dfrac{a_1}{a_2} = \dfrac{b_1}{b_2} \neq \dfrac{c_1}{c_2}$ they are parallel; if all three ratios are equal they coincide. To find $k$, set the ratios equal and check the third.