a-b=-3(a+b)

Solution for a-b=-3(a+b) equation:

Simplifying
a + -1b = -3(a + b)
a + -1b = (a * -3 + b * -3)
a + -1b = (-3a + -3b)

Solving
a + -1b = -3a + -3b

Solving for variable 'a'.

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

Add '3a' to each side of the equation.
a + 3a + -1b = -3a + 3a + -3b

Combine like terms: a + 3a = 4a
4a + -1b = -3a + 3a + -3b

Combine like terms: -3a + 3a = 0
4a + -1b = 0 + -3b
4a + -1b = -3b

Add 'b' to each side of the equation.
4a + -1b + b = -3b + b

Combine like terms: -1b + b = 0
4a + 0 = -3b + b
4a = -3b + b

Combine like terms: -3b + b = -2b
4a = -2b

Divide each side by '4'.
a = -0.5b

Simplifying
a = -0.5b