Answer:
x=31.6
Im not sure since i have not taken a trig course yet, but i know the basics: 
Tangent is opposite over adjacent, and in respect to this angle, (31 degrees) the opposite side is 19 and adjacent is x. 
Tan(31) = 19/x 
Solve for x...use a calculator to find tan(31) 
(Credit to google for the pic)