This is a combining like terms problem. You add or subtract the constants (numbers on the left of the variables (variables = letters like x and y)), but the variables must be the same.
3x -(-3x) = 3x + 3x = 6x
Note : -(-3x) means -3x multiplied by -1, and a negative number multiplied by a negative number becomes positive.
-5y + 6y = y (-5 + 6 = 1....you've got one y left)
The answer is 6x+y