**Step-by-step explanation:**

again, we can transform every equation by doing the same operations on both sides of the "=" sign.

really every operation and function is possible normally, as long as you apply them to both sides.

1.

P = 2(l + b) = 2l + 2b

now to have only b on one side. what do we need to do ?

first, let's move 2l to the other side. we do this by subtracting 2l from both sides :

P - 2l = 2b

ok, and now, how do we get b from 2b ? by dividing by 2 ! again, as always, in both sides :

(P - 2l)/2 = b

b = (161 - 2×45)/2 = (161 - 90)/2 = 71/2 = 35.5 cm

2.

C = 2×pi×r

r should be alone on one side. what do we do ?

let's first divide by 2 (on both sides) :

C/2 = pi×r

ok, and now the same trick just with pi. we divide both sides by pi :

C/(2pi) = r

r = 144.44/(2×3.14) = 144.44/6.28 = 23 cm

3.

m = 5t²

now, we want to get t all by itself on one side.

first we divide (both sides) by 5 :

m/5 = t²

and how do we get t from t² ? by pulling the square root (sqrt()). again, as always, on both sides :

t = sqrt(m/5)

t = sqrt(180/5) = sqrt(36) = 6 seconds