Question 31111
{{{ (5x-7y) + (16x + 8y) }}} since you are adding ... just add the like variables.
{{{ 5x + 16x = 21x }}}
{{{ -7y + 8y = 1y }}}
{{{ 21x + 1y }}}
Problem 2 
{{{ (7n + 2t) + (4n - 3t) }}} drop the parenthesis
{{{ 7n + 2t + 4n - 3t }}} combine like terms
{{{ 11n - 1t }}} 
now just dropping the parenthesis ONLY works if adding.  If subtracting you have to worry about changing the signs of everything AFTER the subtraction