Rectangle

To answer this, you will need to use the pythagorean therom, a²+b²=c².  a and b being the two side lengths, c will be your diagonal.

If a=2.1 and b=.8, the equation will be:

$${{\mathtt{2.1}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{0.8}}}^{{\mathtt{2}}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

Then you square 2.1 and 0.8

$${{\mathtt{2.1}}}^{{\mathtt{2}}} = {\frac{{\mathtt{441}}}{{\mathtt{100}}}} = {\mathtt{4.41}}$$

$${{\mathtt{0.8}}}^{{\mathtt{2}}} = {\frac{{\mathtt{16}}}{{\mathtt{25}}}} = {\mathtt{0.64}}$$

These values substituted into the equation give you:

$${\mathtt{4.41}}{\mathtt{\,\small\textbf+\,}}{\mathtt{0.64}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

When you add together 4.41 and 0.64, you get:

$${\mathtt{4.41}}{\mathtt{\,\small\textbf+\,}}{\mathtt{0.64}} = {\frac{{\mathtt{101}}}{{\mathtt{20}}}} = {\mathtt{5.05}}$$

You can then substitue this value into the equation giving you:

$${\mathtt{5.05}} = {{\mathtt{c}}}^{{\mathtt{2}}}$$

To get the c alone, you can take the square root of both sides, so:

$${{\mathtt{5.05}}}^{{\mathtt{0.5}}} = {\mathtt{2.247\: \!220\: \!505\: \!424\: \!423\: \!2}}$$(x^.5 is the same as taking the sqaure root of x)

The Sqaure root of c² will be c, as the power of 2 will be cancled out by the square root.

So the final result will be 

$${\mathtt{c}} = {\mathtt{2.247\: \!220\: \!505\: \!424\: \!423\: \!2}}$$

So back to your original question.  In context, c is the diagonal of your rectangle.  So when you take the value for c, rounded to the nearest tenth (one decimal place), your answer would be...

$${\mathtt{diagonal}} = {\mathtt{2.2}}$$

Very well presented, Anonymous.....why don't you join up as a member???