Concept:
Two lines are perpendicular if their direction vectors satisfy:
\[
\vec{d_1} \cdot \vec{d_2} = 0
\]
Step 1: Find direction vectors.
From first line:
\[
\vec{d_1} = (2,-1,3)
\]
From second line:
\[
\vec{d_2} = (2,5,a)
\]
Step 2: Apply perpendicular condition.
\[
(2)(2) + (-1)(5) + (3)(a) = 0
\]
\[
4 - 5 + 3a = 0
\Rightarrow -1 + 3a = 0
\Rightarrow a = 1
\]