Answer:
The electromagnetic radiation per-unit- area increase by 42.65 times.
Step-by-step explanation:
The radiation per-unit- area (P) is directly proportional to the fourth power of the temperature. Thus,
P ∝ T⁴
For, when the temperature is raised from T₀ to T₁ , the radiation per-unit- area is related to as:

T₀ = 900 K
T₁ = 2300 K

So,
P₁ = 42.65 P₀
The electromagnetic radiation per-unit- area increase by 42.65 times.