Answer
$Method$ $1$ is much more efficient if we need to find the value of $y$ for several values of $x$.
Work Step by Step
We have the following equation $15x-9y=27$
$x=2$
We have to find the value of $y$.
$Method$ $1$
Let's solve the equation for $y$
$$-9y=27-15x$$
divide all terms by $3$
$$-3y=9-5x$$
$$-y=\frac{9-5x}{3}$$
$$y=\frac{-9+5x}{3}(1)$$
Now substitute $2$ for $x$
$$y=\frac{-9+10}{3}(2)$$
$$y=\frac{1}{3}$$
$Method$ $2$
This time let's directly substitute $x=2$
$$15\times 2-9y=27$$
$$-9y=27-30$$
$$y=\frac{3}{9}$$
$$y=\frac{1}{3}$$
At a glance, $method$ $2$ looks easier to solve; however, when we have to find a value of $y$ for different values of $x$ we have to repeat those steps every time.
With $method$ $1$, we have to calculate equation $(1)$ only once and then substitute different values of $x$.
$Method$ $1$ is much more efficient if we need to find the value of $y$ for several values of $x$.
