Equations, expressions, and assignment-statments
In C++ (and every other programming language I know of) the equal sign is the assignment operator. It assigns the value on the right to the variable on the left. This means that everything on the right must be defined (known). And, the expression on the right must evaluate to a value.
x = 3; //OK
y = x + 3; //OK if x is defined
c = (5.0/9.0 * (f – 32) ); / /OK if f is defined
2 = x; //NO!
x + 2 = 3; // NO!
Note that you might have to put decimals in 5.0/9.0 so that the compiler stores the division-result as a float.