Can someone help me with this please ?

If the triangle is a right triangle, then

       (3x)² + x² = (10)² .

         9x² + x² = 100

             10x²  = 100

                 x²  =  10

                 x  =  √10  =  approx. 3.1622...

If ' x ' is anything less than √10 , then the short sides are too short
to make a right angle at the top, and the angle where they meet
is obtuse.

' x ' has to be greater than  2.5 ... otherwise the two short sides
can't stretch far enough to reach both ends of the long side (10) .

So, if    2.5  <  x  <  √10 , then there is a triangle, and it's obtuse.