The first step to solve this system of equations by substitution is to take one of the equations, preferably the simplest one, and solve it for one variable. In this case, y = 3x + 4 is simpler, and it's already solved for y.
The next step is to replace y in the other equation with what we've found that it equals. So we substitute y in the second equation (2x + 3y = 10) with 3x + 4.
That gives us 2x + 3(3x + 4) = 10.
Looking at the options, we can see that the correct step would be option B, as it corresponds with our substituted equation. So, B. 2x + 3(3x + 4) = 10 is the correct answer.