To find the n^(th) term, we use the formula color (red) (x_n=a+d(n-1)), where n is the term you are looking for, a is the 1^(st) term and d is the difference between terms (it does not vary)
From the arithmetic sequence -3,4,11,18..., we can see that a=-3 and d=7
You can find d by subtracting a number by its precedent =>d=4-(-3)=7 or d=11-4=7 and so on! It's always going to be the same value.
So, x_(15)=-3+7(15-1)=-3+7(14)=-3+98=95
Another way to do this since it's "only" the 15^(th) term is by hands or calculator. They already gave you the four terms, which will help you find the difference "d". Use only the first term "-3" and add 7 until you reach the 15^(th) term.
=>-3+7+7+7+7+7+7+7+7+7+7+7+7+7+7=95
Hope this helps :)
