It can be
t +8 = b
It is a definitional statement for a variable T.
t=x; /* t is the other variable */x=y;y=t;
a variable.
In the expression 5-4t it is t that is the variable
To swap two numbers N1 and N2, using a third variable T... T = N1; N1 = N2; N2 = T;
"t" is a variable. It can mean any number, much like the most commonly used variable "X". 8 + t means that the known number "8" is added to the unknown variable "t". More than likely there it should tell you that 8 + t = ?. In which case you would subtract 8 from the question mark to get t
t=-4
You replace the value of variable "t", then you do the subtraction.
Since T=2pi*sqrt(l/g) and l is the only variable that effects T that is the period it is the length.
t-9
It is called a variable.