K is the Key

( 28/70)^2 = ( 100 / x ) x = area of the bigger triangle x = 625

let $P_1$ be the perimeter of the smaller triangle, $P_2$ be the perimeter of the bigger triangle, $A_1$ be the area of the smaller triangle and $A_2$ be the area of the bigger triangle. The areas of similar plane figures have the same ratio as the squares of any two corresponding lines. Since the perimeter is a linear measure, we have

$\dfrac{A_2}{A_1}=\dfrac{(P_2)^2}{(P_1)^2}$

$\dfrac{A_2}{100}=\dfrac{70^2}{28^2}$

$A_2=\boxed{625}$

Assuming the triangle is equilateral, compute each side of the triangle by dividing each perimeter by $3$ . We know that the area of similar plane figures have the same ratio as the squares of any two corresponding sides. So we have

$\dfrac{A}{\left(\dfrac{70}{3}\right)^2}=\dfrac{100}{\left(\dfrac{20}{3}\right)^2}$

$A=625$ $\text{square centimeters}$