1000=16x^2 idk how to do it

Divide both sides by 16;

$$x^2=\frac{1000}{16}$$

Take the square root of both sides:

$$\\ x=\pm\sqrt{\frac{1000}{16}}\\ x=\pm\frac{\sqrt{1000}}{\sqrt16}\\ x=\pm\frac{\sqrt{1000}}{4}\\ x=\pm\frac{10\sqrt{10}}{4}\\ x=\pm\frac{5\sqrt{10}}{2}$$