What steps can i take to get to the answer? what are the formulas that i'll need.

I believe this is for a calculus class. I assume you have been paying at least a tiny bit of attention in class :)

a)  Finding the RoC of x when y = 50
Take the derivative of 

By Chain Rule, we can drop the 1/2 exponent to the front. The exponent then becomes 1/2 - 1 = -1/2. Then multiply the derivative of the inside function.

Simplifying we get,

Plug y = 50 into your derivative.

around 0.447

b) Find the RoC of the area of triangle ABC when y = 50
Find the derivative of 

The change of area is always 50, regardless of the y value.

c) Find the RoC of theta when y = 50
Let O = theta
sin O = y/100

Take the derivative of O using Chain Rule
The derivative of 
Thus,

Plug y = 50 back in.

which is around 0.0115