Instead of doing the problem for you, I'll do one exactly like yours,
step by step:

if the 4th term of an arithmetic sequence is 21 and the 11th term is 56,
what is the common difference?

__, __, __, 21, __, __, __, __, __, __, 56,...



when n=4, a4=21




when n=11, a11=56




So we have this system



Solve the first equation for a1



Substitute (21-3d) for a1 in








The common difference is 5.

Now do yours the exact same way.

It's not necessary, but if you like you can find the 
first term by substituting d=5 in






and find the arithmetic sequence to be:

6, 11, 16, 21, 26, 31, 36, 41, 46, 51, 56,...
 
Edwin