Demand: L(x, y, m), x -- price of steel, y -- price of aluminium, m - income

(I use Dx to denote the change of x)

Ed = - dL/dx x/L, Ex,y = - dL/dy y/L, Em = - dL/dm m/L

Since

DL = dL/dx Dx + dL/dy Dy + dL/dm Dm

==> DL / L = dL/dx Dx/L + dL/dy Dy/L + dL/dm Dm/L (*)

Since 1) dL/dx Dx/L = dL/dx X/L Dx / X

 2) dL/dx X/L = - Ed,  and

 3) Dx/X is the change of price of steel which is known,

 we can compute dL/dx Dx/L.

Similarly, we can also compute dL/dy Dy/L and dL/dm Dm/L.

So we can get DL / L from (*). Since L is known, we can get DL - the change of demand.

Regards.