Solution to first question.
Let the sides of one triangle be a, b and c and the ratio of the sides of two triangles be 1:k.
So the sides of other triangle are ka, kb and kc.
Perimeter of first triangle is a+b+c.
Perimeter of second triangle is k(a+b+c).
So (a+b+c):k(a+b+c) = 3:4.
Or k is 4/3.
So the sides are also in the ratio 3:4.
So areas are in the ratio 3^2 : 4^2 which is 9:16.
Or the areas are (9/25)*75 = 27 and (16/25)*75 = 48.

So areas are 27 and 48.

Solution to second question:
Ratio of areas is 45:80 = 9:16.
Or sides are in the ratio sqrt(9) : sqrt(16) which is 3:4.
Or perimeters are also in the ratio 3:4.
The sum of perimeters is 35.
Or individual perimeters are (3/7)*35 = 15 and (4/7)*35 = 20.

So perimeters are 15 and 20.