Answer:

unknowns

Answer	Crossword Clue
UNKNOWNS	X's, at times
UNKNOWNS	Solvers' problems
UNKNOWNS	Mathematical variables, often

UNKNOWNS	X and y, in equations
UNKNOWNS	X and Y, in algebra
UNKNOWNS	They're solved for in algebra
UNKNOWNS	Tomb of the ...
UNKNOWNS	X-quantities to be solved
UNKNOWNS	Like beginning actors
UNKNOWNS	They're often represented by x, y and z