The legs of an isosceles triangle have lengths x + 1 and -x + 7 . the base has length 3x - 3. what is the length of the base?
a. 6
b. cannot be determined
d. 4

Since you've told us that this is an isosceles triangle, we know that the 'legs' have equal length.  So we can write

                                                      x + 1 = -x + 7

Add 'x' to each side:                  2x + 1 =       7

Subtract  1  from each side:      2x       =       6

Divide each side by  2 :                x      =       3

Now, if the length of the base is   3x - 3 ,
that length is

                                   3(3) - 3  =  6  .