I am looking to create a ramp like this:

https://imgur.com/a/bpWyrNA

The dimensions are as follows:

38.4654 inches long

22 degree slope

Total vertical climb is 14.75 inches

Stair step beneath the ramp is concrete and 7 inches tall.

All of this will be covered by a 1/4 inch sheet of plywood.

How do I calculate how much weight these 2x6s (framing lumber) can support?

Additionally, how can I better design it so it doesn't slide back, away from the foundation of the house? If I create two triangles on each side to close the gap above the concrete block, will that be sturdy enough for friction to keep it in place with several hundred lbs of load?

Alternatively, I could use the scrap from my angled cuts to add extensions to the bottom base area, and use some rubber chocks to keep it in place.