Question 1000026
This is a system of equations
n=nickels 
q= quarters
You have a total of 21 coins so your first equation is n+q=21
Nickels are each worth .05 cents and quarters are equal to .25 cents so your second equation is .05n+.25q=2.25 
Eliminate one of your variables by multiplying the entire first equation by -.05
-.05n-.05q=-1.05
Add this equation to .05n+.25q=2.25 in order to eliminate your n variable
.2q=1.2
Divide and solve
q=6
You have 6 quarters