Correct.

This is a straightforward calculation of relative error. The only thing about which you need to be careful are the units:

relative error = (absolute error)/(measurement value)

= (2 mm)/(2.6 cm) = (2 mm)/(26 mm) = 2/26 = 0.077 = 7.7% .

That brings up an interesting and helpful subject -- the idea of dimensional analysis. We can use the units of multiplied or divided quantities to check our formulae for accuracy.

Let's use this case as a simple example. Absolute error might be a number of centimetres, seconds, or some other unit. Relative error is always a dimensionless quantity, which means that in the division of the absolute error by the measurement value the units should cancel.

In this question, that meant either changing the measurement value (2.6 cm) into millimetres, or changing the absolute uncertainty (2 mm) into centimetres. Either way, the two quantities would have the same units and division be straightforward.

Dimensional analysis becomes extremely useful in areas of physics like kinematics, where there are lots of equations that would be hard to memorize. You don't need to memorize most elements of these equations. Dimensional analysis will tell you almost everything, except for the numerical factors.

For example, an ubiquitous equation in one-dimensional kinematics is x = x_o + v_oDt + ½a(Dt)² -- work out the units of every quantity, and you'll see they all turn out to be metres. Very handy, and not too hard.