S + L = 30.....S = 30 - L
8.75S + 14L = 283.50
8.75(30 - L) + 14L = 283.50
262.50 - 8.75L + 14L = 283.50
-8.75L + 14L = 283.50 - 262.50
5.25L = 21
L = 21 / 5.25
L = 4......he worked 4 hrs landscaping <===
S + L = 30
S + 4 = 30
S = 30 - 4
S = 26....he worked 26 hrs as a security guard <==