Hey you are on the right track, if you simply sketch out the given lines you will see that a right angled isosceles triangle. The two equal sides of the triangle will be "a"

Now as we know area of a triangle = 1/2*(b)*(h), here b=h=a,

hence 1/2*(a)(a) = 8 ( area is given as 8)

a^2 = 16

so a = 4

Also alternately, if you double the triangle you will get a Square of side "a"

whose area will be +> a*a=2*8

a^2=16

a=4

hope this helps you out

hope that helps you out