inequalities with maximums and minimums!

i dont understand how to find the maximum/minimum of these two equations, help please!

Given the constraints

x+y is less than or equal to 9

3x-y is greater than or equal to 0

x is greater than or equal to 0

y is greater than or equal to 0

the minimum value for the function f(x)=10x + 11y is..?

Given the constraints

3x+4y is greater than or equal to 60

x+8y is greater than or equal to 40

11x+28y is less than or equal to 380

x is greater than or equal to 0

y is greater than or equal to 0

the maximum value of the function f(x)=15x+25y occurs at the corner point..?

explanations please!? (Thinking)