a + b  +c = 10a + b - c = 3 find the value of a + b + 3c

ok if you look at the first to equations carefully you will see that 2 things have changed between the 2. the "+c" is now "-c" and the "10" is now "3"if we think about it you will see that c=7. well how do we know. when c or 7 was taken away from the problem it left you with 3 so that means a+b=3we dont need to know what the exat value of a or b is.so a+b +3c      (3) + 3(7)       3   +  21         24a+b+3c=24