Problem and solution math physic 1

This is one of the math physics exam in Jambi University

$T=4x-(x^2) $ find coldtest and hottest at point $x=5$
 when  $y=0 $
$ dT/dx=4-2x=0$
$x=2$
so we have
$T=4(2)-(2)^2$
T=4


when $ x=5$
we have
 $dT/dy =0$
$T=4(5)-(5)^2$
T=-5


$Hottest point is$ $(2,0) is T=4$
$coldtest point is$ $(5,0) is T=-5$