(speed * 0.2) + 3.5 = ((gap * 0.2) + 3.5) + (gap * grade * 1.8)
Subtract out the 3.5 from both sides.
(speed * 0.2) = (gap * 0.2) + (gap * grade * 1.8)
Multiply both sides by 5 (purely visual).
speed = gap + (gap * grade * 9)
Factor out gap on the RHS.
speed = gap * (1 + (grade * 9))
Divide both sides by the grade factor.
gap = speed / (1 + (9 * grade))