Area=(1/2)(base)(height); 3000=(1/2)b(b+70) where b=base length. 6000=b^2+70b. We can write this 6000+1225=b^2+70b+1225=(b+35)^2. So b+35=sqrt(7225)=85 and b=50mm, making the height 120mm. The hypotenuse is sqrt(50^2+120^2)=sqrt(16900)=130mm. So the perimeter=50+120+130=300mm.