How to solve this problem 800m 35cm - 154 m 49 cm

Can't leave 800m and 35cm dangling with no math operator in between them.  Were you intending multiplication?  Or did you mean 800m PLUS 35cm?

800m plus 35cm - (154m plus 49cm)  = 800.35m - 154.49cm = 645.86 cm

800m 35cm
154m 49cm
subtract

we cant subtract 49 cm from 35 cm...so we will borrow a meter1m = 100 cmso when borrowing, ur problem becomes :

799m 135cm
154m 49cm
subtract
645m 86cm

1m = 100cmso 645m = (645 * 100) = 64500 cmplus the 86 cm...
64500 + 86 = 64586 cm <==

or

1 cm = 0.01 metersso 86cm = (86 * 0.01) = 0.86m plus the 645 meters
645 + 0.86 = 645.86 m <==