Answer
$y=5$
Work Step by Step
Using $y-y_1=\dfrac{y_1-y_2}{x_1-x_2}(x-x_1)$ or the two-point form of linear equations, the equation of the line passing through $(
2,5
)$ and $(
1,5
)$ is
\begin{array}{l}\require{cancel}
y-5=\dfrac{5-5}{2-1}(x-2)
\\\\
y-5=\dfrac{0}{1}(x-2)
\\\\
y-5=0
\\\\
y=5
.\end{array}