Question 1060419
The inside is a trinomial but of known numbers.
{{{sqrt(21)sqrt(21)-2*2*sqrt(21)sqrt(7)+4*sqrt(7)sqrt(7)}}}
{{{21-4sqrt(3*7*7)+4*7}}}
{{{21+28-4*7*sqrt(3)}}}
{{{49-28*sqrt(3)}}}


All that is inside the square root function.
{{{sqrt(49-28*sqrt(3))}}}