Answer:
a = 9
Step-by-step explanation:
First we need to add (x^2+3x) to (3x^2+ax),
(x^2+3x)+(3x^2+ax)
Expand
= x^2+3x+3x^2+ax
Collect the like terms
= x^2+3x^2+3x+ax
= 4x^2 + 3x + ax
= 4x^2+(3+a)x
Equate the solution to 4x^2+12x
4x^2+(3+a)x = 4x^2+12x
Comparing the like terms on both sides
(3+a)x =12x
3 + a = 12
a = 12 - 3
a = 9
Hence the value of a is 9