check the picture below.
since "y" and 3x+45 are both linear angles, then y + (3x + 45) = 180.
but wait a second!! "y" is a corresponding angle to 81°, so y = 81° then, therefore,
[tex]\bf y+(3x+45)=180\implies 81+(3x+45)=180
\\\\\\
126+3x=180
\implies
3x=54\implies x=\cfrac{54}{3}\implies x=18[/tex]
and that can only happen, if c || d.
