Find the lateral area of the right prism with height 0.3 dm, if the base of the prism is: a isosceles trapezoid with bases of 3 cm and 5 cm, and legs of 4 cm;

The lateral area is the product of the height and the perimeter of the base.

The perimeter of the base is the sum of the edge lengths:
  P = 3 cm + 5 cm + 2*4 cm = 16 cm

The height is 0.3 dm = 3 cm.

Then the lateral area id (3 cm)*(16 cm) = 48 cm².

Yes the slope to this is -5