Answer
$\approx \$ 3571.4~$
Work Step by Step
The present value of the income stream after $T$ years can be computed as:
$\int_0^T 250e^{-0.07t} dt=[\dfrac{250e^{-0.07t}}{-0.07}]_0^T\\=\dfrac{250}{0.07}(1-e^{-0.07T}) $
Now, the present value of the entire income stream will be:
$\int_0^{\infty} 250e^{-0.07t} dt=\lim\limits_{T \to \infty}\int_0^T 250e^{-0.07t} \ dt \\=\dfrac{250}{0.07}(1-0) \\ \approx\$ 3571.4~$
