Chemistry
posted by Mely .
The density of iron is 7.87 . What is the mass of a (variable l)* 4.00cm * 1.00 cm block of iron?

7.87 WHAT?
AND you can't have two unknowns. One is mass, the other is l. What is l? 
However, you CAN solve for mass in terms of l.

7.87 g/ml

v = length*4*1 = 4*length
mass = volume x density
mass = 4*length*7.87 = 31.48*length