I'm trying to do math, but i allways fail, what should i do..?

What number should be added to both sides of the equation to complete this square? x^2-10x=7

Three cylinders have a volume of 2836 cm^3. cylinder a has a height of 900 cm. cylinder b has a height of 225 cm. cylinder c has a height of 100 cm. find the radius of each cylinder. use 3.14 as an approximate for π

Researchers were interested in whether relaxation training decreases the number of headaches a person experiences. they randomly assigned 20 participants to a control group or a relaxation training group and noted the change in number of headaches each group reported from the week before training to the week after training. which statistical analysis should be performed to answer the researchers' question?