Close your eyes, AWESOMEEE, if you haven't had Trig, because that's the only wayI know to do this....there are at least two methods......one would be EXTREMELY messy.....I'll opt for the other....
This triangle is iscoceles......so I can use the Law of Sines to find 1/2 of the measure of the apex angle opposite the shortest side....after that....it'll be a breeze.....!!!
So we have
sin 90 /√70 = sin Θ / (1/2)√28 where Θ is the angle we're after. So.....
sin Θ = (1/2) √28 / √70 and using the sine inverse, we have
sin-1 ( √28 / 2 √70) = Θ = about 18.43° and this is 1/2 of the apex angle, so that angle = about 36.87°
Now....we're ready to find the area...so we have
A = (1/2) 70 sin(36.87) = 35 sin (36.87) = 21 sq units