A = bh/2 , with the area 50 m², so:
50 = bh/2
100 = bh
And the height is twice the base:
h = 2b
substitute and solve:
100 = bh
100 = b(2b)
100 = 2b²
50 = b²
b = √50
b = √(25 * 2)
b = 5√2
Now we can solve for h:
h = 2b
h = 2(5√2)
h = 10√2
I would rather keep it in radical form, but if you are told to round to 1 DP, we have:
b ≈ 7.1 m
h ≈ 14.1 m