we will assume that the base is rectangular

w = the width of the base

l = the length of the base

P = 900 ft the perimeter of the base

P = 2(l + w)

2(l + w) = 900

The length of the base measure 70 ft less than three times the width:

l = 3w - 70

plug into 2(l + w) = 900:

2(3w - 70 + w) = 900

by solving we find:

w = 130 ft

click here to see the step by step solution of the equation:

l = 3*130 - 70

l = 320 ft

the dimensions of the base are: 320 ft and 130 ft.