Substitution method involves getting value of one variable from one equation in terms of other variable and putting this in second equation to eliminate and find value of other variable.
Here we have
y=2x + 3
in first equation and putting this in second equation, we get
2x + 3 = 3x + 1 or
2x - 3x = 1 -3 or
-x = -2
i.e.
x = 2
Putting this Vale of x back in first equation, we get