Question 636896
There are 3 equations and 3 unknowns.
That means this is solvable, since 
number of unknowns must equal
number of equations to solve.
------------
(1) {{{ a + b = 15 }}} 
(2) {{{ b - a = 3 }}} 
(3) {{{ a + b - 12 = c }}}
------------------
(1) {{{ a = 15 - b }}}
Substitute this into (2)
(2) {{{ b - ( 15 - b ) = 3 }}}
(2) {{{ b - 15 + b = 3 }}}
(2) {{{ 2b = 15 + 3 }}}
(2) {{{ 2b = 18 }}}
(2) {{{ b = 9 }}}
and, since
(1) {{{ a = 15 - b }}}
(1) {{{ a = 15 - 9 }}}
(1) {{{ a = 6 }}}
Substitute these results into (3)
(3) {{{ a + b - 12 = c }}}
(3) {{{ 6 + 9 - 12 = c }}}
(3) {{{ 15 - 12 = c }}}
(3) {{{ c = 3 }}}
-----------
{{{ a = 6 }}}
{{{ b = 9 }}}
{{{ c = 3 }}}