Question 253199
The formula you need here is
(i) {{{A = (1/2)*ab*sin(C)}}}
where a and b are the sides and C is the "included" angle.
From (i) we have
{{{A = (1/2)*(4)*(5)*sin(120)}}}
sin(120) = sqrt(3) / 2
{{{A = (1/2)*20*sqrt(3)/2}}}
{{{A = 5*sqrt(3)}}}