The half life is 138 days, meaning exactly half of the material will decay after this time. Use this information to find the decay factor
:

Then after 250 days, there will be

grams left.
I'm assuming you meant 1.5 × 10⁴. Remember, when we write in standard form, you need to use ^ to denote it's 'to the power of'. So it would be 1.5 × 10^4.
I'm also assuming you meant in decimal form, because that's already in standard form.
However, the answer is 15,000. This is because you lose one zero when multiplying a decimal with one number after it. For each number after the decimal, you take off a zero. Therefore, the end number is 15,000.