Finding the Freundlich adsorption isotherm from the graph of log(x/m) v/s log(p)
One version of Freundlich adsorption isotherm equation is:
$$ \frac xm = Kp^{\frac12}, $$
which can also be written as:
$$ \log \frac xm = \log K + \frac12\log p $$
This is a straight line equation of type $( y = c + mx)$ as given in the question. However, I think the question has made a mistake saying the slope is $2$, but instead it should be $\frac12$. Accordingly, the equation with given numeric values should be:
$$ \log \frac xm = \log 3 + \frac12\log p $$
Or without logarithm:
$$ \frac xm = 3p^\frac12 $$
On plugging $p = 4$, You'll get:
$$ \frac xm = 3 \times 4^\frac12 = 3 \times 2 = 6 $$
Hence, you get the given answer.