In this proble we have an expression for each angle of the triangle so we can us the property that the sum of the internal angles of a triangle must be equal to 180 so the expression will be:
[tex]5x+18+3x+2+x+16=180[/tex]So we can solve the expression for x so:
[tex]\begin{gathered} 9x+36=180 \\ 9x=180-36 \\ x=\frac{144}{9} \\ x=16 \end{gathered}[/tex]Now with the value of x we can calculate the angle D that will be equal to:
[tex]\begin{gathered} \angle D=3(16)+2 \\ \angle D=48+2 \\ \angle D=50 \end{gathered}[/tex]