Answer:
(x + 1) (x + 13)
Multiply one variable each from the first parentheses to the two variables on the second parentheses.
Ex. (1 + 2) (x + y) = 1 · x + 1 · y + 2 · x + 2 · y = 1x + 1y + 2x + 2y = 3x + 2y
So you do the same process as in the example.
(x + 1) (x + 13)
x · x + x · 13 + 1 · x + 1 · 13
x² + 13x + x + 13
= x² + 14x + 13
