Question 78584
You could try this:
{{{sqrt(s-16) + sqrt(s) = 4}}} Subtract{{{sqrt(s)}}} from both sides.
{{{sqrt(s-16) = 4-sqrt(s)}}} Now square both sides.
{{{s-16 = 16-8sqrt(s)+s}}} Subtract s from both sides.
{{{-16 = 16-8sqrt(s)}}} Subtract 16 from both sides.
{{{-32 = -8sqrt(s)}}} Divide both sides by -8.
{{{4 = sqrt(s)}}} Finally, square both sides.
{{{16 = s}}}

Check: 
{{{sqrt(s-16) + sqrt(s) = 4}}} Substitute s = 16
{{{sqrt(16-16) + sqrt(16) = 4}}}
{{{sqrt(0) + sqrt(16) = 4}}}
{{{4 = 4}}}