7 cm x 3 sides = 21 cm perimeter.
It is: 3*7 = 21cm
Perimeter = sum of the lengths of sides = 5* 7 cm = 35 cm
Perimeter = 8*7 = 56 cm
Two sides of a triangle are not sufficient to determine its area.
The perimeter is the addition of the all the sides of the shape. For example if a rectangle has two sides which are 4cm and two sides which are 7cm, then the perimeter will be 4cm+4cm+7cm+7cm. The perimeter would therefore be 22cm. The area of a shape is gotten by multiplying the length and breadth of the shape. For example, in a rectangle witrh the same measurements above, the area will be 4cm x 7cm and that would 28cm. You have to use the square sign but I can't do that here.
if it is a scalene triangle yes scalene triangle have no congruent sides and angles
No. The two shortest sides, 2cm and 4cm, add up to 6cm. 6 cm is shorter than 7cm, so those three sides will not make a triangle.