A block is acted upon by forces of 430N, 100N, 450N, and 800N. Determine the net force on the block.

We'll consider forces to the right as positive and forces to the left as negative.

$$F_{net,x} = 750 \, \text{N} - 430 \, \text{N}$$ $$F_{net,x} = 320 \, \text{N} \text{ (to the right)}$$

We'll consider forces downwards as negative.

$$F_{net,y} = -100 \, \text{N} - 800 \, \text{N} - 1000 \, \text{N}$$ $$F_{net,y} = -1900 \, \text{N} \text{ (downwards)}$$

The magnitude of the net force is found using the Pythagorean theorem:

$$F_{net} = \sqrt{F_{net,x}^2 + F_{net,y}^2}$$ $$F_{net} = \sqrt{(320 \, \text{N})^2 + (-1900 \, \text{N})^2}$$ $$F_{net} = \sqrt{102400 \, \text{N}^2 + 3610000 \, \text{N}^2}$$ $$F_{net} = \sqrt{3712400 \, \text{N}^2}$$ $$F_{net} \approx 1926.76 \, \text{N}$$

The angle $\theta$ with respect to the positive x-axis (horizontal to the right) is given by:

$$\theta = \arctan\left(\frac{F_{net,y}}{F_{net,x}}\right)$$ $$\theta = \arctan\left(\frac{-1900 \, \text{N}}{320 \, \text{N}}\right)$$ $$\theta = \arctan(-5.9375)$$ $$\theta \approx -80.44^\circ$$

This means the force is $80.44^\circ$ below the horizontal, in the fourth quadrant.

The magnitude of the net force is $\boxed{1926.76 \, \text{N}}$ at an angle of $\boxed{80.44^\circ \text{ below the positive horizontal axis}}$.