Question 7668
I'll assume you meant
2a+b=14
3a+b=4

Solving by substitution means finding b in terms of a in one equation and using that value of b in the second equation.

b = 14 - 2a [subtracting 2a from both sides]
So the second equation is
3a + (14 - 2a) = 4  [replacing b with its value 14 - 2a]
a + 14 = 4  [simplifying 3a-2a]
a = -10 [subtracting 14 from both sides]

Now put the value of a back into the first equation.
2(-10) + b = 14 [substituting the known value of a]
-20 + b = 14 [multiplying]
b = 34 [adding 20 to both sides]

Checking:
2(-10) + 34 = 14
3(-10) + 34 = 4