Question 1115103
S should be expressed in displacement not in distance since the problem is involved in a vector quantity. Anyway if this is meant by the problem and no direction is involved, as magnitude is indicated alone then, distance would be accepted for this case now.

we know that {{{v = sqrt(2as)}}}. We'll isolate s in the given formula.

1st - square both sides: {{{v^2 = (sqrt(2as))^2}}} which leaves us {{{v^2 = 2as}}}
2nd - divide both sides by 2a(we apply division PE) {{{(v^2)/2a = 2as/2a}}}

where {{{s = v^2/2a}}} 

3rd - Substitute {{{s = ((80ft/s)^2) / (2(20ft/s^2))}}}
{{{s = (6400 ft^2/s^2)/(40ft/s^2)}}}

s = 160 ft