Is the sentence a simple or a compound sentence?
stone bridges are strong but are costly to build.
a. compound sentence
b. simple sentence

This is a compound sentence, because there are two independent clauses here: 1. Stone bridges are strong + but 2. (strong bridges) are costly to build.
The conjunction 'but' connects two independent clauses, and a compound sentence contains at least two independent clauses, whereas a simple sentence has only one. 

y = 3x + 62

Step-by-step explanation:

Using the points :

(0, 62) and (6, 80)

Equation of trend line is in the form:

y = mx + c

Where m = slope ; c = intercept

Slope (m) = (y2 - y1) / (x2 - x1)

m = (80 - 62) / (6 - 0)

m = 18 / 6

m = 3

Intercept, c = value of y when x = 0

From (0, 62)

x = 0 ; y = 62

Then c = 62

Hence,

y = 3x + 62