The way you wrote it could have 2 meanings:
Sum of twice a and 4 could be:
2a+4 or 2(a+4) which will give different results.
In the first case:
2a+4
a= 4/2
a= 2
So the sum would be 2*2+4 = 4+4 = 8
In the second case:
2(a+4) = 2a + 2*4 = 2a + 8
a= 8/2
a=4
2(4+4) = 2*8 = 16
