Another way to substitute a variable by a value, besides with the | operator or the subst command, is with something akin to functional notation. You can follow an expression or expression name with equalities of the form variable=value.
| Expr:=x+2*y+3*z | 
then:
| subst(Expr,[x=1,y=2]) | 
or:
| Expr | x=1, y=2 | 
| Expr(x=1,y=2) | 
| 
 | 
| (h*k*t^2+h^3*t^3)(t=2) | 
| 
 |