A rhombus is a square but turned so it looks like a diamond. This means all the sides are equal. With this information we can build an equation to solve for x.
3x - 10 = 6x - 19
1) Subtract 3x from both sides.
-10 = 3x - 19
2) Add 19 to both sides.
9 = 3x
3) Divide both sides by 3.
3 = x
x =3.