You can sum like terms. Two terms can be summed if they are both constants (i.e. pure numbers) or if they involve the same powers of the same variable.
In this case, 3x and 13x can be summed because they both involve 'x' alone. The sum is
[tex] 3x+13x = (3+13)x = 16x [/tex]
As for the other two terms, you simply have to sum two numbers, as usual:
[tex] -9-6 = -15 [/tex]
So, the final answer is
[tex] 16x-15 [/tex]
