A skier from rest skis straight down a slope 50 m long in 5 s. What is the magnitude of acceleration of the skier?

Use the formula

acceleration=\Delta v/t

where t is time and \Delta v the variation in velocity.

How would u solve it though?

The skier starts from rest (thus 0 m/s), and skies 50 m. Let be a its acceleration. His speed at time t is
v(t)=at, and the distance travelled is
s(t)=\int_0^t v(x)dx=\frac{1}{2}at^2

You know from problem data that after 5s he has travelled 50m, thus
50m = \frac{1}{2}a(5s)^2
and solve in a to get the acceleration (result 4m/s^2).