A regular pyramid means its base is a regular polygon (equal sides).
The base must have 5cm side since it's a square w/ area 25cm².
+ Lateral Area (4 AΔ)
150 = 25 + LA
LA = 125 = 4 AΔ
AΔ = 125/4
Recall that AΔ = 1/2 bh
h = 25/2 = 12.5cm
Therefore, the slant height is 12.5 cm