$\text{ The reflection of the orthocentre of the triangle formed by the points }(2,3),(2,4)\text{ and }$$(5,4)\text{ in }x\text{-axis is. }$

$\text{ The given points are }A(2,3),B(2,4),C(5,4)$

$\text{ Since the }x\text{-coordinates of two points }A(2,3),B(2,4)\text{ are same, the line joining }A,B\text{ is verticale line}$

$\text{ Since }y\text{-coordinates of two points }B(2,4),C(5,4)\text{ are same. the line joining }B,C\text{ is }$

$\text{ horizontal line, so that the triangle }ABC\text{ is right angled triangle with vertex at right angle is B}\left(2,4\right)$

In right angled triangle, the ortho centre is vertex at right angle. 
Hence, the ortho centre is  $\left(2,4\right)$ .
The reflection of a point $\left(x,y\right)$ in $x-$ axis is   $\left(x,-y\right)$
Therefore, the reflection of $\left(2,4\right)$ in $x-$  axis is   $\boxed{\left(2,-4\right)}$