Problem A
A rectangle has equal diagonals
So 9x + 20 = 10x + 8 Subtract 8 from both sides.
9x + 20 - 8 = 10x
9x + 12 = 10x Subtract 9x from both sides.
12 = 10x - 9x
12 = x Answer <<<<<
Problem B
The diagonals of a rhombus bisect the angle vertex that the diagonal is connect between. All that language says is that
2x + 15 = 5x + 3 Subtract 2x from both sides.
15 = 5x - 2x + 3
15 = 3x + 3 Subtract 3 from both sides.
15 - 3 = 3x
12 = 3x Divide by 3
12/3 = x
x = 4 <<<< Answer B
When x = 4 the two angles are equal.