ANSWER
The value of x is,
[tex]17.5[/tex]
EXPLANATION
We were given that the opposite angles of a parallelogram are
[tex]3x - 20[/tex] and [tex]x + 15[/tex].
The opposite angles of a parallelogram are equal.
We can write the equation below in x and use to find the value of x.
[tex]3x - 20 = x + 15[/tex]
We group like terms to obtain,
[tex]3x - x = 15 + 20[/tex]
This implies that,
[tex]2x = 35[/tex]
We divide both sides of the equation by 2 to obtain,
[tex]x = 17.5[/tex]
