With an equation you must isolate x or whatever the variable is. Operations are the things used to make the difference like +,-,x,/.
A great example of this is the following
4x-12=0
First add 12 to both sides making the equation go to
4x=12
now divide 4 from both sides making
x=3
To find the answer, set up an equation:

Using this set up we can cross out the units that are on opposite sides of the proportion such as miles and hours, leaving us with the desired units of meters and seconds. Then calculate m/s by multiplying 45 by 1000 meters and then dividing that value by 3600 seconds.
The resulting answer is 45mph = 12.5 m/s