A standard glucose solution of 0.5mN concentration gives an absorbance
of 0.35 and this lies on the linear part of the standard curve. What
is the concentration in a sample giving an absorbance of 0.21? and if
the above sample was from a solution of hydrolysed maltose, what would
be the concentration of maltose?

The only way to answer this question easily is if you tell us the path
length of the detector.

Once you have the path length, you can calculate the molar
absorbtivity coefficient (or extinction coefficient) at any wavelength
using the Beer-Lambert law:

Absorbance = extinction coefficient * concentration * path length

Once you have the molar absorbtivity coefficient for your first value
on the standard curve, you can figure out the concentration of your
sample giving an absorbance of 0.21 by plugging it into the equation
and solving for concentration.

Since maltose is a glucose dimer, the concentration of maltose would
be

total concentration - (1/2)glucose concentration

I am assuming you have some way of telling what the initial
concentration of maltose was.

Enjoy.

You don't need to know the path length.

You know the different absorbance values .35 and .21

Absorbance = extinction coefficient * concentration * path length

.35 = extinction coefficient * 0.5mN * path length
.21 = extinction coefficient * X * path length

.35/.21 = (extinction coefficient * 0.5mN * path length)/(extinction
coefficient * X * path length)

the extinction cooefficients and path lengths cancel out and you are
left with  a ratio

.35/.21= .5mN/X

Solve for X

X = (.21*.5mN)/.35;