Explanation:
1.
6, 8, 13 can be the side lengths of a triangle in general. the rule there is that the sum of any 2 sides must be larger than the remaining 3rd side.
6+8 = 14 > 13 ok
6+13 = 19 > 8 ok
8+13 = 21 > 6 ok
but they cannot be the side lengths in a right-angled triangle.
because a right-angled triangle has to satisfy the Pythagoras rule :
c² = a² + b²
c is the Hypotenuse (the side opposite of the 90° angle and is the longest of the 3 sides). a and b are the legs (the 2 sides that connect at the 90° angle).
in our case that would mean
13² = 6² + 8²
169 = 36 + 64 = 100
169 is NOT 100, so NO, 6, 8, 13 cannot be the side lengths of this triangle.
2.
remember, the sum of all angles in a triangle is always 180°.
one angle is 90°.
then angle 1 is 40°.
that makes angle 2 = 180 - 90 - 40 = 50°.
NO, it is not an isoceles triangle, because an isoceles triangle means its legs are equally long. and that means that the angles of the legs with the baseline are equally large.
but in 90°, 40°, 50° there is no pair of equal angles.