First you find the number of sides of the polygon using the sum of its interior angles.
180 (n-2) = 1260
180n - 360 = 1260
180n = 1260 + 360
Now that you have the number of sides, determine the measurement of each side. Since it is a regular polygon, they have equal sides.
9s = 45 cm
From there, you use the formula: area= s^2 (n) / 4 tan (180/n) wherein s is the measurement of the side and n the number of sides.
area= 5^2 (9) / 4 tan (180/9)
= 225 / 4 tan 20
= 154.55 cm^2
Therefore, your area is 154.55 cm squared.
I hope this helped :)