We need to set up a fraction multiplication with the units we want to cancel on the top left and bottom right. Since we want to miles to cancel we set it up like this
45mi/hr 1.61km/hr
----------- x ------------ =
1 1mi/hr
miles and hours cancel from the top right and bottom left leaving only km/hr as our units. Then we just multiply the fraction
45 time 1.61 is 72.45
1 times 1 is 1
72.45/1=72.45km/hr