90дм 4мм-8м 34см=

To calculate the expression 90 дм 4 мм - 8 м 34 см, you have to keep the units consistent. Let's first convert everything to centimeters.

There are 10 centimeters (cm) in a decimeter (dm), 10 millimeters (mm) in a centimeter, and 100 centimeters in a meter (m).

So the expression in centimeters would be:

90 дм 4 мм = (90 дм to см) + (4 мм to см) 90 дм = 90 * 10 см = 900 см 4 мм = 4 / 10 см = 0.4 см

Therefore, 90 дм 4 мм is 900 см + 0.4 см = 900.4 см

Now for the second part: 8 м 34 см = (8 м to см) + 34 см 8 м = 8 * 100 см = 800 см

Thus, 8 м 34 см is 800 см + 34 см = 834 см

Now, let's subtract the second quantity from the first one in centimeters:

90 дм 4 мм - 8 м 34 см = 900.4 см - 834 см = 66.4 см

So, the result is 66.4 centimeters.