Explanation:
did you also try Pythagoras, the first thing you always remember and try for right-angled triangles ?
you remember :
c² = a² + b²
c is the Hypotenuse (the baseline, it is opposite of the right angle - if it is a right-angled triangle, it is the longest side).
a and b are the legs.
the perimeter is the sum of all 3 sides.
22 = AB + BC + AC = 8 + 5 + AC
AC = 22 - 8 - 5 = 9 cm
so, now, if it is a right-angled triangle, the 3 sides must satisfy the Pythagoras equation (remember, in that case also the longest side must be the Hypotenuse) :
9² = 8² + 5²
81 = 64 + 25 = 89
that is wrong, so it is NOT a right-angled triangle.