Solve the following pair of linear equations by the method of substitution: $5x - 3y = 1$ and $2x + 5y = 19$.

After $x$ years,the sum of the ages of Sachin and Sehwag will be $y$ years,then the sum of the ages before $3$ years was $\ldots \ldots \ldots \ldots$ years.

Obtain the solution of the following pair of equations by elimination method:
$\frac{11}{y} - \frac{7}{x} = 1, \, \frac{9}{y} - \frac{4}{x} = 6$; $x \neq 0, \, y \neq 0$

$A$ boat goes $32 \, km$ upstream and $36 \, km$ downstream in $7$ hours. In $9$ hours,it can go $40 \, km$ upstream and $48 \, km$ downstream. Determine the speed of the stream and that of the boat in still water. (Speed of boat in still water is more than the speed of the stream of river.)

Comparing the equation $x=4$ with the standard form $ax+by+c=0$,we get $b=\ldots \ldots \ldots$

The graph of the pair of equations $2x + y = 7$ and $5x - 2y = 4$ is: