Answer
$R=850 \text{ ohms}$
Work Step by Step
As resistance $R$ and temperature $T$ vary directly so
$R=kT$ (where $k$ is a constant of proportionality)
We plug in the known values to obtain:
$646\text{ ohms}=k(190K)$
$\implies k=\frac{646\text{ ohms}}{190K}=3.4\frac{ohms}{K}$
Thus,
$R=\left(3.4\frac{\text{ ohms}}{K}\right)T$
Now we can find the resistance for $T=250K$ as
$R=3.4\frac{\text{ ohms}}{K}(2500K)
\\R=850 \text{ ohms}$
