To check whether the given linear equation 3x+6=15 is true for x=3, substitute x=3 in the qiven eqaution and check whether L.H.S=R.H.S

For the given equation, 
L.H.S = 3x+6
Substitute x=3 in the given equation,
Therefore, L.H.S = 3x(3)+6
L.H.S = 9+6
L.H.S = 15
From the given equation, R.H.S = 15
It is clear that, L.H.S = R.H.S for x=3.
Therefore, the given linear equation 3x+6=15 is true for x=3.