break it up as 3 separate equations:
(1) a+3c=5
(2) 3+3*7=b+c
(3) c+3c=15
solve equation 3: 4c=15 so c = 3.75
solve equation 2 for b (since we now know c): 3 +21=b+c >> 24=b+c >> b=24-c
plug c into this equation: b=24-3.75 >> b=20.25
solve equation 1 for a (since we know c): a=5-3c
and plug c into this equation: a=5-3(20.25) >> a=6.25
the solution takes the form of an (x,y,z) ordered pair... (6.25, 20.25, 3.75)