.subs plugs numbers or other expressions in. .evalf is a decimal.

Goal

Substitute Nairobi units=12 into 10.5*x.

import sympy as sp
x = sp.symbols('x')
print((10.5 * x).subs(x, 12))

import sympy as sp
x = sp.symbols('x')
print(sp.sqrt(2).evalf(10))

import sympy as sp
x, y = sp.symbols('x y')
print((x + y).subs({x: 1, y: 4}))

import sympy as sp
x = sp.symbols('x')
print(sp.pi.evalf())