You can see from the diagram that it is a right angled triangle with opposite side AB and adjacent side BC. Using SOH, CAH, TOA trig identity to get the length of AB. According to TOA;
tan 35° = opposite/adjacent
tan 35° = AB/BC
tan 35° = AB/100
AB = 100tan35°
AB = 100 * 0.7002
AB = 70.02m
Hence the distance across a lake between A and B is approximately 70m