For the first problem, we know that all the angles in a triangle add up to 180˚. That means: x + ( 2x + 5 ) + ( 3x + 7 ) = 180.

Since addition is associative, x + ( 2x + 5 ) + ( 3x + 7 ) = 180 is the same as x + 2x + 5 + 3x + 7 = 180.

Combining like terms, we get: 6x + 12 = 180.

After we move 12 to the other side so the x is iscolated, we get: 6x = 168.

Dividing each side by 6 gets us to: x = 28, so the answer to the first problem is D.

I hope this helped! I might've made a mistake, so if you catch one, please lmk! <3