(7a+3)+(-a-5)=15

Solution for (7a+3)+(-a-5)=15 equation:

Simplifying
(7a + 3) + (-1a + -5) = 15

Reorder the terms:
(3 + 7a) + (-1a + -5) = 15

Remove parenthesis around (3 + 7a)
3 + 7a + (-1a + -5) = 15

Reorder the terms:
3 + 7a + (-5 + -1a) = 15

Remove parenthesis around (-5 + -1a)
3 + 7a + -5 + -1a = 15

Reorder the terms:
3 + -5 + 7a + -1a = 15

Combine like terms: 3 + -5 = -2
-2 + 7a + -1a = 15

Combine like terms: 7a + -1a = 6a
-2 + 6a = 15

Solving
-2 + 6a = 15

Solving for variable 'a'.

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

Add '2' to each side of the equation.
-2 + 2 + 6a = 15 + 2

Combine like terms: -2 + 2 = 0
0 + 6a = 15 + 2
6a = 15 + 2

Combine like terms: 15 + 2 = 17
6a = 17

Divide each side by '6'.
a = 2.833333333

Simplifying
a = 2.833333333