If the point $(2, 3)$ lies on the graph of the equation $3x + ay = 18$,find the value of $a$.

Find the value of $k,$ if $(2,5)$ is a solution of $3x - 4y = k$.

For what value of $c,$ the linear equation $2x + cy = 8$ has equal values of $x$ and $y$ for its solution?

For the equation $F = (9/5)C + 32$,$F$ and $C$ are numerically equal when $F = \ldots$

At what point does the graph of the linear equation $x+y=5$ meet a line which is parallel to the $y$-axis,at a distance $2$ units from the origin and in the positive direction of the $x$-axis?

For the equation $5x + 3y = 90$,if $x = 12$,then $y = \dots$