The americans with disabilities act requires that wheelchair ramps have a slope no greater than 1/12 what is the minimum horizontal length a 2 foot high ramp can have ?

The "slope" means how far you rise when you travel 1 unit horizontally.
"Steeper" means you rise higher for each horizontal distance ... more slope.
A slope of 1/12 means 12 horizontal units raise you one unit higher.
12 horizontal inches raise you 1 inch higher.
1 horizontal foot raises you 1 inch higher.
12 horizontal feet raises you 1 foot higher.

In order to rise 2 feet, you'd have to cover (2 x 12) = 24 horizontal feet
on a 1/12 slope.