Use the distributive property to distribute the 5b by the 6b and -10 terms.
Here are two examples of the distributive property:
a(b - c) = ab - ac
6(3b - 8) = 18b - 48
11b = 5b + 6b. You added 5b and 6b. You need to multiply them together:
5b * 6b = 30b^2
Also, the -10 inside the parenteses must also be multiplied by 5b, so 5b * 10 = 50b
5b(6b - 10) = 30b^2 - 50b