Can someone explain why 33 and 35 are these answers?

For #32,
P=2L+2W
Subtract 2W from both sides, and swap left and right
2L = P-2W
Divide by 2
2L/2=(P-2W)/2
L = P/2 - 2W/2
L=P/2 - W

For #35
Most of the expenses are in fractions (of the original amount, A), so they can be added:
A/4 + A/5 + 2A/5 + 750  = A
add the fractions, with a common denominator of 20,
5A/20 + 4A/20 + 8A/20 +750  = A
(5A+4A+8A)/20 +750 = A
17A/20 + 750 = A

Now subtract 17A/20 from both sides and swap left and right
A - 17A/20 = 750
(3/20)A = 750

Multiply both sides by 20/3  (to make one unit of A on the left)

(3/20)*(20/3) A = 750*20/3
A =250*20=5000