Let us take the second equation first
2a + 2b = 6
Dividing both sides by 2 we get
a + b = 3
a = 3 - b
Putting the value of a in the first equation we get
3a + 4b = 9
3(3 - b) + 4b = 9
9 - 3b + 4b = 9
b = 9 - 9
= 0
Now putting the value of b in the second equation we get
a + b = 3
a + 0 = 3
a = 3
So the value of the unknown variable a is 3 and the value of the unknown variable b is 0.