You have the following expression:
2.5 x 10⁴ - 1.5 x 10³
in order to simplify the previous expression, consider that the factors with base 10 have to have the same exponent. You can write the second term as a factor with 10⁴ if you write 1.5 as 0.15 (if you increase the exponent you move the decimal point to the left). Then, you can subtract the coefficients of the term and put the same factor x 10⁴:
Then, you have:
1.5 x 10³ = 0.15 x 10⁴
2.5 x 10⁴ - 0.15 x 10⁴ = 2.35 x 10⁴
Hence, the asnwer is:
2.35 x 10⁴