Answer
$-\frac{5\pi}{4}$ radians
Work Step by Step
To convert from degrees to radians, the measurement in degrees must be multiplied by $\frac{\pi}{180}$.
Therefore
$-225°= -225° \times \frac{\pi}{180}$
$=-\frac{225\pi}{180}$
$=-\frac{5\pi}{4}$ radians