On planet Mars, A pendulum has a length of 4L and period of 5T.what is the acceleration due to gravity ?

For a simple pendulum, period

T=2*π*sqrt(L/g)
Substitute T=5T, L=4L
5T=2*π*sqrt(4L/g)
=>
square both sides and solve for g:
25T=4π²(4L)/g
g=16Lπ²/25T
where L and T are as given in the question.