We are given the ratio as BC:AC:AB = 4:3:5
Middle side is AC and its measure is 12 cm.
Total ratio = 4+3+5 = 12
So that means :

x=48
So perimeter =48 cm
Now let us find the other two sides: AB and BC
BC = 4x/ 12 = (4*48)/12= 16 cm
AB = 5x/12 (5*48)/12 =20 cm
Figure shows the actual representation of the triangle.
Area:
Area of triangle is given as :

here base is 16 cm and height is 12 cm, plugging in formula,

Area = 96 square cm
Height of Hypotenuse = AB = 20cm