Which function represents a vertical stretch of an exponential function?
a. f(x)=3(1/2)^x
b. f(x)=1/2(3)^x
c. f(x)=(3)^2x
d. f(x)=3^(1/2x)

Concept:  
always remember the general rule which is 
y = cf(x): for c> 1, stretches the graph of f vertically by a factor of c.
y = (1/c)f(x): for c> 1, compresses  the graph of f vertically by a factor of c.

in our question, the first case(as mentioned above)  happens, so the correct answer is  a: f(x)=3(1/2)^x.     
see the attached graph.

Since he starts with (10x+7) dollars and is buying two pairs of jeans that each cost (4x-5) dollars you should subtract the cost of the jeans from his starting amount.

(10x+7)-2(4x-5)=
10x+7-8x+10=
2x+17