Derivative of square root (x^2+16).

I get up to 1/2(x^2+16)^-1/2(2x) but im stuck from there

Solve the following pair of equations for x and y: ax/b_ by /a= a +b; ax - by = 2ab

Astudent must have an average on five test that is greater than it equal to 80% but less than 90% to receive a final grade of b. devon's greades on the first four test were 78% 62% 91% and 80% what range if grades on the fifth test would give him a b in the course? ( assuming the highest grade is 100%)

Breana is selling tickets to a choral performance. on the first day of ticket sales , she sold 3 senior citizen tickets and 1 child ticket for a total of $38. breana took in $52 on the second day by selling 3 senior citizen tickets and 2 child tickets. what is the price of a senior citizen ticket?