You cannot, because you have no information on the shape: is it a 2-d triangle or a 3-d cuboid?
Draw a right angled triangle with legs of 4cm and 6cm
3cm
3cm + 4cm + 3cm + 4cm = 14 cm
# #
An isosceles triangle.
Yes
5cm
5cm
No because the given dimensions don't comply with Pythagoras' theorem for a right angle triangle.
A scalene triangle
the equation for area is area = length * width. area = 3cm*4cm. Area = 12cm^2
1/2*4*3 = 6 square cm
5 cm
This is the simplest Pythagorean Triple (52 = 32 + 42) so it's a right(-angled) triangle
The area of triangle is : 8.0
The area is 12 cm2.