SymPy express variable in terms of another

This is how you can express this equation in terms of x:

In [1]: from sympy import *

In [2]: x, y = symbols('x, y')

In [3]: expr = 2*x - 7*y

In [4]: solve(expr, y)
Out[4]: [2*x/7]

This works because if the solve() function is presented with something that is not a full equation, it assumes that the provided expression is equal to zero. In other words, writing

expr = 2*x - 7*y

above is equivalent to writing

expr = Eq(2*x - 7*y, 0)

which would tell SymPy that

2x - 7y = 0.