For this case we have the following equation:
 w = || F || • || PQ || costheta
 Where,
 || F ||: force vector module
 || PQ ||: distance module
 costheta: cosine of the angle between the force vector and the distance vector.
 Substituting values:
 w = (60) * (100) * (cos (45))
 w = 4242.640687 lb.ft
 Answer:
 The work done pushing the lawn mower is: 
 w = 4242.640687 lb.ft