Answer:
124 mm is the longest side
Step-by-step explanation:
5x + 3 + 7x + 5 + 12x -20 = 276
24x -12 = 276
24x = 288
x = 12
1st side = 5x+3 =5(12)+3=63 mm
2nd side = 7x + 5=7(12) + 5=89 mm
3rd side = 12x -20= 12(12) -20=124 mm
