(21-2y)-(15-6y)=0

Solution for (21-2y)-(15-6y)=0 equation:

Simplifying
(21 + -2y) + -1(15 + -6y) = 0

Remove parenthesis around (21 + -2y)
21 + -2y + -1(15 + -6y) = 0
21 + -2y + (15 * -1 + -6y * -1) = 0
21 + -2y + (-15 + 6y) = 0

Reorder the terms:
21 + -15 + -2y + 6y = 0

Combine like terms: 21 + -15 = 6
6 + -2y + 6y = 0

Combine like terms: -2y + 6y = 4y
6 + 4y = 0

Solving
6 + 4y = 0

Solving for variable 'y'.

Move all terms containing y to the left, all other terms to the right.

Add '-6' to each side of the equation.
6 + -6 + 4y = 0 + -6

Combine like terms: 6 + -6 = 0
0 + 4y = 0 + -6
4y = 0 + -6

Combine like terms: 0 + -6 = -6
4y = -6

Divide each side by '4'.
y = -1.5

Simplifying
y = -1.5