For the equation $ax + by + c = 0$,if $c = 0$,then its graph is a line passing through the $\ldots \ldots \ldots$.

Draw the graph of the equation represented by the straight line which is parallel to the $x$-axis and is $4$ units above it.

The autorickshaw fare in a city is charged $Rs. 10$ for the first kilometer and $Rs. 4$ per kilometer for the subsequent distance covered. Write the linear equation to express the above statement and draw the graph of the linear equation.

The graph of the equation $3x + 7y = k$ is a line passing through the origin. Then,$k = \ldots$

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

If $(2,3)$ is a solution of the equation $7x - 3y = a$ and $(a, a+1)$ is a solution of the equation $2x + y = b$,find the values of $a$ and $b$.